Add logging of agent heartbeats

Add logging of agent heartbeats

When troubleshooting problems with cluster it would be
very convenient to have information about agent heartbeats
logged with some searchable identifier which could create
1-to-1 mapping between events in agent's logs and server's logs.

Currently agent's heartbeats are not logged at all on server side.
Since on a large cluster that could create too much logging
(even for troubleshooting cases), it might make sense to make
this configurable both on neutron-server side and on agent-side.

DocImpact

Change-Id: I0a127ef274a84bba5de47395d47b62f48bd4be16
Closes-Bug: #1452582