Improve compatibility between Keystone V2 and V3

To improve compatibility between Keystone V2 and V3, this renames the
client @property in both of them from self.client_v2 and self.client_v3
to just self.client.

This allows both of them to be used by third party contrib plugins in a
seamless manner.

Closes-Bug: 1289528
Co-Authored-By: Richard Lee <rblee88@gmail.com>
Change-Id: I60412ecb1665232ffeeeb869fb00d368604601d2
Merge "Improve compatibility between Keystone V2 and V3"