Implement provision_process() across the board. [#3012030]

Now that provision_process() exists, we need to use it.

provision_process() can replace:

drush_shell_exec()
provision_backend_invoke()
?

I think we will end up with a provision_process_drush() command that will be more like provision_backend_invoke() so it will be easy to trigge drush commands.

There's still some more work to do to, related to backed options passing back from drush calls.

Marking this major because this is important work. This function will be the separation point we need to break away from the drupal entanglement.

I tried doing this in the $server->shell_exec() method, it seemed to work but failed the tests. I shouldn't have committed this to the main branch. http://cgit.drupalcode.org/provision/commit/?id=01c2e1d091ecf9aac4731c1f...

Here's helmo's reverrt:
See http://cgit.drupalcode.org/provision/commit/?id=b0ee01ebc4189a3aeb0f6f8e...