Make Keystone client pluggable

Similarly to how the engine client works, this makes it possible to
switch out the builtin keystone client with a custom - interface
compatible - plugin replacement.

Co-Authored-By: Anderson Mesquita <andersonvom@gmail.com>
Partial-Bug: #1274201
Change-Id: If14ff4b34b90f46d0f468fd669dcc2fdc36c8c97
Merge "Make Keystone client pluggable"