Add compose type to release for images

Move getting of the precomputed value to a single place in Compose
class. The value now has format `date[.type_suffix].respin`.

Resolves: rhbz#1319924
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/helpers.py

@@ -26,6 +26,7 @@ class DummyCompose(object):
         self.compose_respin = 0
         self.compose_id = 'Test-20151203.0.t'
         self.compose_label = None
+        self.image_release = '20151203.t.0'
         self.ci_base = mock.Mock(
             release_id='Test-1.0',
             release=mock.Mock(