Restore quotas if enqueuing job to Ganeti fails
When creating a server, we commit the VM model in DB and reserve the corresponding quotas in Quotaholder before enqueuing the OP_INSTANCE_CREATE job (because messages from dispatcher may arrive before committing). In case enqueuing job to Ganeti fails we must restore the quotas to their previous value. This commit achieves this by issuing a "delete" commission for the VM. Also, instead of deleting the VM entry, we mark the VM as deleted, in order to keep the relation with the Quotaholder serial.