Conductor de-registers on shutdown

Conductor service should de-register itself upon shutdown.
This will cause RPC requests to immediately stop routing to it
rather than there being an unnecessary period of malfunction
before the next heartbeat cycle.

A new unit test (test_stop_unregisters_conductor) is added to
validate the de-registration of the conductor. Test hostname
values are coalesced into a single variable for consistency.

Co-Authored-By: David Shrewsbury <shrewsbury.dave@gmail.com>

Change-Id: I1feea2c0165cde36f2abb95c3fa3484c73dfd81c
Merge "Conductor de-registers on shutdown"