Improved handling of Rack state

- Set to 'provisioning' when Heat stack is CREATE_IN_PROGRESS
  or UPDATE_IN_PROGRESS

- Set to 'active' when Heat stack is UPDATE_COMPLETE or CREATE_COMPLETE

- Set to 'error' when Heat stack is UPDATE_FAILED or CREATE_FAILED
Merge pull request #70 from mfojtik/rack_state

Improved handling of Rack state