Drop `vagrant ssh -c` & communicate directly via SSH.

This was done by calling `vagrant ssh-config` after the instance is
created which returns the SSH configuration required to make further
connections. This configuration is saved off into the state bag just
like any other SSH-based driver.

Thanks to an upstream API change
(https://github.com/opscode/test-kitchen/commit/a37c2c61f341a88e840438c31136eac49453ca0b)
SSH configuration can come from .kitchen.yml or from the saved instance
state.
Release 0.7.4.