Allow heat standalone to work devstack stack

For functional testing of heat-standalone it is desirable for
heat to orchestrate on the rest of the cloud which is brought up
by devstack. This change makes the following changes to enable
this when HEAT_STANDALONE=True:

- Don't register the orchestration endpoint or create any dedicated
  heat accounts
- Install and configure the heat keystone V2 auth plugin instead of
  the default v3
- set heat.conf [clients_heat] url so that heat can call its own
  API when no orchestration endpoint is registered
- Modify create_userrc.sh to set the required heat client env
  variables to work with the standalone heat

Change-Id: Idae33bf1a9d550e2575e6390d2d7c8d3b94c401d
Merge "Allow heat standalone to work devstack stack"