title: Creating a Glue job with AWS CDK (python) fails
body: <p>I'm using Python wrappers for CDK to create a Glue job. The <code>command</code> attribute requires an object of type <code>IResolvable | Job­Command­Property</code>. I tried to put a <code>JobCommandProperty</code> object here but I'm getting an exception.</p>
  
  <p>I created a <code>JobCommandProperty</code> object. I was looking for a <code>.builder()</code>function somewhere (similar than in the Java API), but couldn't find one.</p>
  
  <pre class="lang-py prettyprint-override"><code>from aws_cdk import (
      aws_glue as glue,
      aws_iam as iam,
      core
  )
  
  class ScheduledGlueJob (core.Stack):
  
      def __init__(self, scope: core.Construct, id: str, **kwargs) -&gt; None:
          super().__init__(scope, id, **kwargs)
  
          policy_statement = iam.PolicyStatement(
                  actions=['logs:*','s3:*','ec2:*','iam:*','cloudwatch:*','dynamodb:*','glue:*']
              )
  
          policy_statement.add_all_resources()
  
          glue_job_role = iam.Role(
              self,
              'Glue-Job-Role',
              assumed_by=iam.ServicePrincipal('glue.amazonaws.com')
          ).add_to_policy(
              policy_statement
          )
  
          job = glue.CfnJob(
              self,
              'glue-test-job',
              role=glue_job_role,
              allocated_capacity=10,
              command=glue.CfnJob.JobCommandProperty(
                  name='glueetl',
                  script_location='s3://my-bucket/glue-scripts/job.scala'
              ))
  </code></pre>
  
  <p>The error message is this:</p>
  
  <pre><code>$cdk synth
  Traceback (most recent call last):
    File "app.py", line 30, in &lt;module&gt;
      glue_job = ScheduledGlueJob(app, 'Cronned-Glue-Job')
    File "/Users/d439087/IdeaProjects/ds/test_cdk/.env/lib/python3.7/site-packages/jsii/_runtime.py", line 66, in __call__
      inst = super().__call__(*args, **kwargs)
    File "/Users/d439087/IdeaProjects/ds/test_cdk/glue/scheduled_job.py", line 33, in __init__
      script_location='s3://my-bucket/glue-scripts/job.scala'
    File "/Users/d439087/IdeaProjects/ds/test_cdk/.env/lib/python3.7/site-packages/jsii/_runtime.py", line 66, in __call__
      inst = super().__call__(*args, **kwargs)
    File "/Users/d439087/IdeaProjects/ds/test_cdk/.env/lib/python3.7/site-packages/aws_cdk/aws_glue/__init__.py", line 2040, in __init__
      jsii.create(CfnJob, self, [scope, id, props])
    File "/Users/d439087/IdeaProjects/ds/test_cdk/.env/lib/python3.7/site-packages/jsii/_kernel/__init__.py", line 208, in create
      overrides=overrides,
    File "/Users/d439087/IdeaProjects/ds/test_cdk/.env/lib/python3.7/site-packages/jsii/_kernel/providers/process.py", line 331, in create
      return self._process.send(request, CreateResponse)
    File "/Users/d439087/IdeaProjects/ds/test_cdk/.env/lib/python3.7/site-packages/jsii/_kernel/providers/process.py", line 316, in send
      raise JSIIError(resp.error) from JavaScriptError(resp.stack)
  jsii.errors.JSIIError: Expected 'string', got true (boolean)
  </code></pre>
  
  <p>Maybe someone has a working CDK (python) example to create a <code>CfnJob</code>object?</p>