deprecate apiclient package

Based on our discussion at the Kilo summit, we indicated that this
module would be marked deprecated but retained until the new SDK is
completed.

https://etherpad.openstack.org/p/kilo-oslo-library-proposals

Change-Id: Id5e624995cd8fb9da4265db7d84eac6b699679b2
Merge "deprecate apiclient package"