You need to supply auth_url to authenticate using Keystone. Otherwise,
auth_token, user_id, project_id and mistral_url provided will be
directly transmitted to HTTPClient.
Also:
* renamed input_auth_token as auth_token
* optimized few checks
* added checks to prevent header update when not needed
* simplified API test setUp
Change-Id: I1b34282956d21a1809ec37b0368fbb68aa113bea
Implements: blueprint mistral-optional-keystone-authentication
df55840009