title: terraform get isn't compatible with git ssh
body: <p>I want to load module from gitlab through ssh.
  I have configured ssh correctly so that "git push" with ssh works</p>
  
  <p>module is constructed with the following code</p>
  
  <pre><code>module "elasticsearch_security_group" {
    source = "git::ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-security-group.git"
    name = "elasticsearch_security_group"
    vpc_id = "${var.vpc_id}"
    ingress_cidr_blocks = ["0.0.0.0/0"]
    create = "true"
  }
  </code></pre>
  
  <p>but "terraform get " doesn't work with git ssh</p>
  
  <pre><code>2018/07/30 13:40:38 [INFO] Terraform version: 0.11.7  41e50bd32a8825a84535e353c3674af8ce799161
  2018/07/30 13:40:38 [INFO] Go runtime version: go1.10.1
  2018/07/30 13:40:38 [INFO] CLI args: []string{"/usr/local/bin/terraform", "get", "--update=true"}
  2018/07/30 13:40:38 [DEBUG] Attempting to open CLI config file: /home/ec2-user/.terraformrc
  2018/07/30 13:40:38 [DEBUG] File doesn't exist, but doesn't need to. Ignoring.
  2018/07/30 13:40:38 [INFO] CLI command args: []string{"get", "--update=true"}
  - module.elasticsearch_security_group
    Updating source "git::ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-security-group.git"
  2018/07/30 13:40:38 [TRACE] module source: "git::ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-security-                                       group.git"
  2018/07/30 13:40:38 [TRACE] detected module source "git::ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-s                                       ecurity-group.git"
  2018/07/30 13:40:38 [DEBUG] fetching module from git::ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-secu                                       rity-group.git
  2018/07/30 13:40:38 [DEBUG] fetching "git::ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-security-group.                                       git" with key "1.elasticsearch_security_group;git::ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-securit                                       y-group.git"
  2018/07/30 13:40:38 [ERR] Checkpoint error: Get https://checkpoint-api.hashicorp.com/v1/check/terraform?arch=386&amp;os                                       =linux&amp;signature=1876a657-05db-8573-ee03-ee17a78287f6&amp;version=0.11.7: Forbidden
  2018/07/30 13:40:38 [DEBUG] plugin: waiting for all plugin processes to complete...
  Error loading modules: error downloading 'ssh://git@ssh.git.tech.rz.cb.de:Username/tf-module-aws-security-group.g                                       it': /usr/bin/git exited with 128: Cloning into '.terraform/modules/31a9f10a7ca5618dcd541d5e2c188286'...
  ssh: Could not resolve hostname ssh.git.tech.rz.cb.de:Username: No address associated with hostname
  fatal: Could not read from remote repository.
  
  Please make sure you have the correct access rights
  and the repository exists.
  </code></pre>
  
  <p>Did I miss something here ?</p>