Remove instance_get() from conductor API

The instance_get() method of the conductor API is no longer
used.  Remove it from the client code and leave a note on the manager
side for it to be removed in the next major version bump.

This is cleanup, but part of a blueprint being completed during
feature freeze.  It allows us to drop more code in the rpc version
bump.

Part of blueprint rpc-major-version-updates-icehouse

Change-Id: Id2ed674c2d4c491bc7b0b0c79a64bef84fb05db4
Merge "Remove instance_get() from conductor API"