Controller maintenance mode

Currently there is no functionality built into controller which gives user ability to safely undertake repairs in operating system, its services etc. These activities can influence running Openstack environment thus all Openstack services should be stopped.

We need to introduce a maintenance mode for controllers which means all services except crucial ones (like networking, sshd, etc.) will be stopped. That will provide user a safe way to provide necessary changes on the system and ensure that no Openstack operation will be handled on this node.