AUTO-816: Disconnecting from cass on shutdown

LoggingCQLClient.disconnect is implemented in silverberg 0.1.6. Hence
the version upgrage
service super methods called - thanks @glyph
trying disconn after supervisor jobs are completed
Merge branch 'master' into auto-816-cass-disconn

Conflicts:
	otter/tap/api.py

Combined health checker info
removing duplicate multiservice creation
Merge branch 'auto-816-cass-disconn' into cass-disconn-try

Conflicts:
	otter/tap/api.py
no need for nofify_on_jobs_completion

Can use stopService instead
cassandra_disconnect implemented

that disconnects after supervisor completes stopping
updated tests for cassandra_disconnect
Not using supervisor.stopService - thanks @cyli

when disconnecting from cassandra since it assumes that stopService is
idempodent which we don't know.
Merge pull request #512 from rackerlabs/auto-816-cass-disconn

AUTO-816: Disconnecting from cass on shutdown