Implement an ssh_config passthrough param

- provide a way for users of Vagrant.session context manager to specify ssh
  configuration to be used.

  This change was driven from the need to change the user being used to SSH
  into the vagrant VM during fabric tasks. This change also facilitates being
  able to delete the user that the Vagrantfile specifies to use when connecting.
Make sure to copy the config dict in ssh context.
Merge pull request #1 from jong/feature/custom-ssh-config

Implement an ssh_config passthrough param