Feature/google support

GCE support.  some notes:

   * disk creation/confirmation happens first in the create task
   * a data disk of configurable size is mounted to /data if given
   * requiretty sudoers config is disabled if present
   * selinux is set to permissive if present
   * provisioner requires an ssh user/key, which must be configured in the GCE project metadata.  the user can be defined in the plugin ssh_username field (optional, takes precedence), or else defaults to the image/server value
   * GCE default network is firewalled externally on all ports except 22 by default.  some initial firewall configuration or custom network definition outside of loom will likely be needed.
   * the ability to disable ephemeral IP assignment will require fog upgrade to 1.23.  will do in a future PR.

to support the use case of defining username in the image like the other providers