Fixes #164 Enable a single environment variable to be passed

[proffalken]

OK, so this doesn't fix it really, however it does allow a single ENV variable to be set:

    driver_config:
      image: centos:7
      platform: centos
      provision: true
    # Don't use this as it installs EPEL6 on CentOS7
      require_ansible_repo: false
      install_epel_repo: true
      custom_env: container docker
    # Make sure that we swap out the fakesystemd for the real one
      provision_command:

results in

-----> Starting Kitchen (v1.4.2)
-----> Cleaning up any prior instances of <apache2-centos-71>
-----> Destroying <apache2-centos-71>...
       Finished destroying <apache2-centos-71> (0m0.00s).
-----> Testing <apache2-centos-71>
-----> Creating <apache2-centos-71>...
       Sending build context to Docker daemon 57.34 kB
       Sending build context to Docker daemon 
       Step 0 : FROM centos:7
        ---> 0f73ae75014f
       Step 1 : ENV container docker

My ruby is decidedly rusty so I've completely failed to be able to parse multiple custom_envs into a hash and then back into the Dockerfile, but I figure this is a good start and I'm willing to accept any suggestions about turning

- custom_env:
   - container: docker
   - role: apache

into

ENV container docker
ENV role apache

[coderanger]

This is now supported via run_options and --env.