Enforces the use of Credentials (part1)

Multiversion auth part5

Refactor tenant isolation to use Credentials instead of username,
password and tenant_name, but keeps the same interface for now,
so test don't have to be changed.

Tenant isolation is  still using v2 identity clients to generate
credentials, even when auth_version is set to v3.
This changes prepares the migration to v3.

Partially implements: bp multi-keystone-api-version-tests

Change-Id: If2da9d869af417b6cd8f6a4c48d710e817c76402
Merge "Enforces the use of Credentials (part1)"