Savanna: add API client and tests for plugins
Savanna: add API client and tests for plugins

Change-Id: I1af1a3e6dbaeec2e1c7a1b9db9fc009822196f9b

openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario,openstack/sahara-scenario,openstack/sahara-tests,openstack/sahara-tests,openstack/sahara-scenario