Adds Images API tests

* Adds new images API tests in tempest/tests/images
* Adds tempest.openstack.ServiceManager class to deal with non-Compute API
  clients
* Adds an [image] section to the configuration file and manager
  that deals with Image API stuff
* Updates the tools/conf_from_devstack script to write an image
  section to the generated tempest.conf

This is all in preparation for removing the functional integration
tests from Glance and putting them in Tempest...

Change-Id: I6caf50e5cab97794204472151acc88fcdd0fc224
8 files changed