Add a test documentation section to the docs

This commit adds a new section the HACKING and REVIEWING docs. It
explains the guidelines around inline documentation when
contributing new tests.

Change-Id: Ib56723d0c748baf41439975102d1363f81085f50
Migrate scenario utils to tempest client

All scenario tests have been migrated to tempest client.
Migrate scenario utils as well, so that we may drop
the OfficialClientManager completely.

Change-Id: I3ec80de961ba289734f9bc811e4789e7d1b5903c
Partially-implements: bp/tempest-client-scenarios
Merge "Add a test documentation section to the docs"
Merge "Migrate scenario utils to tempest client"
Merge branch 'master' of https://github.com/openstack/tempest