Refactored cinder tempest tests to cinder_tempest_plugin

* As per the current codebase of cinder tempest plugin, it is getting
  dumped in the cinder/tests and the package name is setup.cfg is
  cinder_tempest_plugin which does not exists. So i refactored to
  give a proper tempest plugin shape.

Change-Id: Ia78ea53a99923844f40fb4e9fe22ec0da4b0c335
16 files changed