Adding trust delegation and removal for proxy users

Changes
* adding a generic keystone client creation function
* adding a function to create proxy user clients
* renaming admin client functions for clarity
* adding token delegation/removal during proxy user creation/deletion
* adding a configuration option for proxy user trust roles

Change-Id: I362a27c1b1d0c5b81d6fbbb91fc0689e339f0076
Partial-implements: blueprint edp-swift-trust-authentication
Merge "Adding trust delegation and removal for proxy users"