Add a new column and indexes to agent_heartbeats

Includes a new index on instance_id
Added a deleted and deleted_at column
Includes an index for deleted too
Includes in call to find all by version
Includes call in _delete_resources()
Fixed AgentHeartBeat delete call
Corrected downgrade script as per SlickNik
Needs the save method because of how we are calling create
Reworked migration to create table from scratch
Renamed migration from 026 --> 027
Exception handling, added not null to updated_at col
Renamed migration from 027 --> 028

Change-Id: Ie0568850856c32fdbf9495c496339730d6b5beef
Implements: blueprint upgrade-guestagent
Merge "Add a new column and indexes to agent_heartbeats"