Add support for provision_command

[berniedurfee]

The kitchen-docker driver has a 'provision_command' attribute that allows you to specifically set the command executed when provisioning the Chef client. This is much more flexible than just setting the URL for the script and would really help those of us trapped behind a corporate firewall.

[sethvargo]

Thanks @cobzilla. This will be included as part of test-kitchen/test-kitchen#329.

[ozbillwang]

Seems still in progress.

[Lirt]

For anybody looking for hotfix, you can follow this guide http://www.elmund.io/2015/07/05/run-a-script-on-a-test-kitchen-vm-before-converge-starts/

Basically if you specify driver in .kitchen.yml like this:

platforms:
   - name: fedora-27
     driver:
       provision: True
       vagrantfiles:
         - fedora27_provision.rb

And then you specify provisioning file fedora27_provision.rb like code below, you can do preprovisioning steps:

Vagrant.configure(2) do |config|
  config.vm.provision "shell", inline: <<-SHELL
     sudo dnf install -y python2
  SHELL
end