Replace trivial docstring instances of tenant

The only changes that made sense to do in the client were three
instances of the word tenant in some doc-strings. So they were
changed. Other than those, the occurrences of 'tenant' in the
codebase relate to keystone's v2.0 API.

Partially implements: blueprint replace-concept-of-tenants-for-projects

Change-Id: If4984d30b552a1eccf5ab927559584dfeebc9593