Add CiviCRM support to Golden Contrib

We'd like to add hosting_civicrm to the Aegir distro. This will allow CiviCRM to be hosted out-of-the-box by Aegir, and also make it eligible for support under our new Aegir Support Services.

In preparation of doing so, we're going to merge provision_civicrm into hosting_civicrm as per #2300537: Merge Provision extensions into Hosting modules. This means the upgrade path could get a bit tricky. That is, an existing deployment will have provision_civicrm in ~/.drush. When the hosting_civicrm feature is enabled, the drush/ dir will be included via our registry. This should result in Drush failing due to duplicated function definitions, etc.

The upgrade path will thus have to involve removing ~/.drush/provision_civicrm before the upgrade. For the .deb package, we figure the best route is to follow the example set by drush_make. That is, simply abort (with a helpful warning) if /var/aegir/.drush/provision_civicrm exists.