builder: use cloud api

Composer now[1] has integrated the koji API into the "cloud API"
and thus we can use this more general purpose and powerful API
instead of using the specialized koji API endpoint.
Adapt the request and response structures as well as the unit
tests to use that.

[1] PR #2214, commit 11e2ae45284bfb0d89ef1c1e0d2aa4ae230ea573

test/unit/test_cli.py

@@ -88,7 +88,7 @@ class TestCliPlugin(PluginTest):
         ]
 
         expected_args = ["name", "version", "distro",
-                         ['qcow2'], # the default image type
+                         ['guest-image'],  # the default image type
                          "target",
                          ['arch1']]