Updated images fixture

* Updated images fixture to use servers client and servers behaviors
* Want to use servers behavior methods inside images tests:
* - To create active servers
* - To create active image from servers
* Imported and used servers_config, flavors_config and
* compute_endpoint_config
* Created ComputeIntegrationFixture for using compute clients and
* behaviors

Change-Id: I89d3d389b8ed27d46e64f7a8138dccc3f885eed3