Fix a race condition in agents status update code

Fix a race condition in agents status update code

Code handling agents status updates coming via RPC checks,
if a corresponding entry for the given (agent_type, host)
pair already exists in DB and updates it. And if it doesn't
exist, a new entry is created.

Without a unique constraint this can cause a race condition
resulting in adding of two agent entries having the same value
of (agent_type, host) pair.

Note, that it's already not allowed to have multiple agents of
the same type having the same host value, but currently it's
enforced only at code level, not at DB schema level, which
effectively makes race conditions possible.

Closes-Bug: #1254246

Change-Id: I1ebaa111154b3d6b34074705b579097ab730594c