ubuntu: start gating on Ocata packages

It seems like Canonical now provides packaging for Ocata release:
http://ubuntu-cloud.archive.canonical.com/ubuntu/dists/xenial-updates/ocata/main/

Let's use them.

Change-Id: I83c7c909074698ef87b52eb5e2b1cec1fb4cc26d

manifests/repos.pp

@@ -4,9 +4,12 @@ class openstack_integration::repos {
     'Debian': {
       include ::apt
       class { '::openstack_extras::repo::debian::ubuntu':
-        release         => 'newton',
+        release         => 'ocata',
+        repo            => 'proposed',
         package_require => true,
-        uca_location    => $::uca_mirror_host,
+        # use the mirror again when this patch is merged:
+        # https://review.openstack.org/404759
+        # uca_location    => $::uca_mirror_host,
       }
       # Ceph is both packaged on UCA & ceph.com
       # Official packages are on ceph.com so we want to make sure