Fetch next deployable release basen on OPENSTACK_RELEASE var

Right now we have mitaka and mitaka+UCA releases which both uses
Ubuntu as operation system. We want to predict "which release will be
taken for next operations".
'octane upgrade-env' now requires release id so this patch is required
for further changes
https://review.openstack.org/#/c/358881/

Change-Id: Icc4c2f534a13e11995129bad62b4fc985633fb34

fuelweb_test/models/fuel_web_client.py

@@ -2366,7 +2366,8 @@ class FuelWebClient29(object):
             if (release["id"] > release_id and
                     release["operating_system"] ==
                     release_details["operating_system"] and
-                    release["is_deployable"]):
+                    release["is_deployable"] and
+                    OPENSTACK_RELEASE in release["name"].lower()):
                 return release["id"]
 
         return None