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.

this is the disk leak issue i brought up and we decided against it for the reason that someone can detach disks outside of loom management.  disks can not be deleted before the server.  one possible solution is to attempt to detach first but i still need to store all this disk state info in the payload, which is ugly (we need a higher level management layer).