Obtain a token right before API test execution

Currently authentication token for Heat API tests is obtained once during
test suite initialization.

This patch introduces Gabbi fixture and corresponding changes to tests which
assures a token is requested before test execution

Change-Id: I0e2a044c383a68b2e7b019d0028fbb81d833ef56
Closes-Bug: https://mirantis.jira.com/browse/PROD-19362
7 files changed