Remove aggregate_get_by_host() from conductor API

The aggregate_get_by_host() 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: Ib28fab80adbe796ecaded040d4c33ea05659e509
Merge "Remove aggregate_get_by_host() from conductor API"