Issue title: tomcat上部署dubbox消费者jar包冲突问题
 Issue body: dubbox 2.8.4 依赖 spring-web（版本 ：3.2.9.RELEASE） 会和tomcat中的jar包冲突。
导致消费者无法跑在tomcat上，大家是如何解决的

错误内容：
Cannot cast org.springframework.web.SpringServletContainerInitializer to javax.servlet.ServletContainerInitializer

详细内容
严重: Failed to detect ServletContainerInitializers for context with name []
java.io.IOException: java.lang.ClassCastException: Cannot cast org.springframework.web.SpringServletContainerInitializer to javax.servlet.ServletContainerInitializer
	at org.apache.catalina.startup.WebappServiceLoader.loadServices(WebappServiceLoader.java:205)
	at org.apache.catalina.startup.WebappServiceLoader.load(WebappServiceLoader.java:157)
	at org.apache.catalina.startup.ContextConfig.processServletContainerInitializers(ContextConfig.java:1575)
	at org.apache.catalina.startup.ContextConfig.webConfig(ContextConfig.java:1281)
	at org.apache.catalina.startup.ContextConfig.configureStart(ContextConfig.java:889)
	at org.apache.catalina.startup.ContextConfig.lifecycleEvent(ContextConfig.java:386)
	at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:117)
	at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:90)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5479)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1574)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1564)
	at java.util.concurrent.FutureTask.run(FutureTask.java:262)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.ClassCastException: Cannot cast org.springframework.web.SpringServletContainerInitializer to javax.servlet.ServletContainerInitializer
	at java.lang.Class.cast(Class.java:3176)
	at org.apache.catalina.startup.WebappServiceLoader.loadServices(WebappServiceLoader.java:197)
	... 15 more

 Comments: 
 Comment 0: pom文件依赖列表
  <dependencies>
		<dependency>
			<groupId>com.pursuit</groupId>
			<artifactId>auth-rbac-common</artifactId>
			<version>0.0.1-SNAPSHOT</version>
		</dependency>
		 
		<!-- Spring 补充-->
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-webmvc</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-beans</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context-support</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-aspects</artifactId>
		</dependency>
		<!-- 文件上传组件 -->
		<dependency>
			<groupId>commons-fileupload</groupId>
			<artifactId>commons-fileupload</artifactId>
		</dependency>
		<!-- Apache工具组件 -->
		<dependency>
			<groupId>org.apache.commons</groupId>
			<artifactId>commons-lang3</artifactId>
		</dependency>
		<dependency>
			<groupId>org.apache.commons</groupId>
			<artifactId>commons-io</artifactId>
		</dependency>
		<!-- 单元测试 -->
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
		</dependency>
		<!-- JSP相关 -->
		<dependency>
			<groupId>jstl</groupId>
			<artifactId>jstl</artifactId>
			<scope>provided</scope>
		</dependency>
		<dependency>
			<groupId>javax.servlet</groupId>
			<artifactId>servlet-api</artifactId>
			<scope>provided</scope>
		</dependency>
		<!-- 日志处理 -->
		<dependency>
			<groupId>org.slf4j</groupId>
			<artifactId>slf4j-log4j12</artifactId>
		</dependency>
		<dependency>
		    <groupId>com.alibaba</groupId>
		    <artifactId>fastjson</artifactId>
		    <version>1.2.4</version>
		</dependency>
		<dependency>
			<groupId>org.apache.zookeeper</groupId>
			<artifactId>zookeeper</artifactId>
		</dependency>
		<dependency>
		    <groupId>com.101tec</groupId>
		    <artifactId>zkclient</artifactId>
		</dependency>
		<!-- dubbo -->
		<dependency>
			<groupId>com.alibaba</groupId>
			<artifactId>dubbo</artifactId>
		</dependency>
	</dependencies>
 Comment 1: 由于 dubbox 2.8.4 依赖 javax.servlet-api 与tomcat 冲突
变更  javax.servlet-api 作用域  <scope>provided</scope>
		<dependency>
		    <groupId>javax.servlet</groupId>
		    <artifactId>javax.servlet-api</artifactId>
		    <version>3.1.0</version>
		    <scope>provided</scope>
		</dependency>

注意：之前错误找不到原因，主要是因为dubbo依赖的是 javax.servlet-api  而不是 servlet-api
		<dependency>
			<groupId>javax.servlet</groupId>
			<artifactId>servlet-api</artifactId>
			<scope>provided</scope>
		</dependency>

{'source': 'GithubIssues', 'repo_name_with_owner': 'dangdangdotcom/dubbox', 'issue_id': 201898028, 'repo_stars': 4944.0}