Create a test token generator and use it

All the clients are currently storing samples of keystone tokens so that
they can use them in testing. This is bad as they are often out of date
or contain data that they shouldn't.

Create a V2 Token generator and make use of that for generating tokens
within our tests.

Change-Id: I72928692142c967d13391752ba57b3bdf7c1feab
blueprint: share-tokens