Fix SUSE gate overrides outside gates

When storing the nodepool information task into suse mirror, the
resulting variable could be storing ansible information, not only
stdout. That could lead to malformed urls for the variable, when
the check is skipped outside gates.

See the value of a repo, containing ``opensuse_mirror`` here:
"{'skipped': True,
  'skip_reason': 'Conditional result was False',
  'changed': False}/distribution/leap/42.3/repo/oss/"

Change-Id: Idfd3c233c2239b642154960b536abe1f7fca7942

common-tasks/test-set-nodepool-vars.yml

@@ -87,7 +87,7 @@
     echo "${NODEPOOL_OPENSUSE_MIRROR}"
   args:
     executable: /bin/bash
-  register: opensuse_mirror
+  register: _opensuse_mirror
   delegate_to: localhost
   when:
     - ansible_pkg_mgr == 'zypper'
@@ -97,7 +97,7 @@
 
 - name: Set a fact to override the openSUSE mirror URL when in nodepool
   set_fact:
-    opensuse_mirror: "{{ opensuse_mirror.stdout }}"
+    opensuse_mirror: "{{ _opensuse_mirror.stdout }}"
     # OpenStack infra doesn't mirror OBS repos so use the upstream one
     opensuse_mirror_obs: "http://download.opensuse.org"
   when: