Add ability to enable static credentials mode in tempest

For some test cases (Barbican, Ldap) we need to have an ability
to set tempest to work with static credentials.

Change-Id: If93445cc27b087d4f3a1a0dbb456b3c7acef0923
Related-Prod: https://mirantis.jira.com/browse/PROD-18731
3 files changed