TaskManager: Only support single node locking

The multi-node locking is not used and there's no plan to use it.
Removing it allows us to clean up a lot of code.

The TaskManager tests are refactored to not use the DB.

Removed 'node' from get_node_mac_addresses() in driver/utils as it's
no longer needed.

Partial-Bug: 1312632

Change-Id: Ie3577ba150df6c84fc34a4b08775e5011f8925cb
Merge "TaskManager: Only support single node locking"