Add support for a post_create_command

[hectcastro]

There is already a pre_create_command that executes a custom command prior to the vagrant up command. This pull request adds support for a post_create_command as well.

I believe supplying something like vagrant ssh -c 'some command' to post_create_command could resolve #78.

[sethvargo]

This will be captured as part of test-kitchen/test-kitchen#329