Manage and unmanage shares

Manila does not have possibility to import/register already existing shares on storages to its DB.

We agreed that manage/unmanage functionality will be implemented initially only for driver_manage_share_servers=False driver's, because this is the easier case to implement.

Use cases:
1. As admin I want to add existing NAS to manila (as driver_manage_share_servers=False backend) and import existing shares to manila using "manila manage" command.
2. As admin I want to unmanage share from manila to perform some maintenance.