Use Discovery fixtures for auth token tests

These structures can be provided by keystoneclient Fixtures. The current
examples are actually incorrect as there are no links provided by the v2
discovery response.

Whilst at it we may as well use the built-in JSON handling of the
requests-mock library rather than encode it ourselves.

Change-Id: Icf227971458c92db0b0c77b5e3d08d1a9fec795e