Model Location: fix validators
Maintenance mode: optimalizations

Columns maintenance_lock and maintenance_lock_reason were added
to all maintainable models. Values of these columns are set
when (un)locking objects. Lock/unlock operations are now slower,
but it makes reading lock info much more simpler and without additional
database queries.
MaintenanceLock.lock!: add missing line of code
Maintainable: fix maintenance_check!
Allow reading of location, node, vps, vps_config by users
MaintenanceLock.unlock!: do not unlock if object still has master lock