Remove OfficialClient dependency from HACKING.rst

This commit migrates OfficialClientTest to ScenarioTest about
TestVolumeBootPattern in HACKING.rst. We should not have such an example
in the HACKING.rst because we don't use OfficialClientTest class in the
scenario tests anymore.

Partially-implements: bp:tempest-client-scenarios

Change-Id: Id95d011f3d080effc7bfee6232578451f7ba72d9
Merge "Remove OfficialClient dependency from HACKING.rst"