service_statuses updated_at Field Not Set

The service_statuses updated_at value is never set due to
inconsistent column naming ('updated' is automatically set in the
base model). The result is the value remains NULL throughout the
instance creation lifecycle, despite multiple row updates.

Fix: override save() and set updated_at (following the pattern
established in agent_heartbeats)

Change-Id: I71fea5001fa1868d221f50c34605022e6a1022bd
Closes-Bug: #1199518
Merge "service_statuses updated_at Field Not Set"