Set current ceph dashboard mgr backend fact

Due to a potential issue related to regex_replace combinated to an
ansible varialble value rendering, it's recommended setting the
fact and simplify the Ceph command that is supposed to set the backend.

Change-Id: I56d8862522fe82cb9f2373574ee067fbe4cae98d

tripleo_ansible/roles/tripleo_cephadm/tasks/dashboard/configure_dashboard_backends.yml

@@ -21,17 +21,20 @@
   become: true
   delegate_to: "{{ dashboard_backend }}"
 
+- name: Check the resulting mgr container instance
+  set_fact:
+    current_mgr: "{{ ceph_mgr.stdout | regex_replace('^-n mgr.(.*)(?P<inst>) -f (.*)+$', '\\1') }}"
+
 - name: Check the resulting mgr container instance
   debug:
-    msg: "{{ ceph_mgr.stdout | regex_replace('^-n mgr.(.*)(?P<inst>) -f (.*)+$', '\\1') }}"
+    msg: "{{ current_mgr }}"
   when: tripleo_cephadm_verbose | bool
 
 - name: config the current dashboard backend
   command: |
     {{ tripleo_cephadm_ceph_cli }} config set \
-    mgr mgr/dashboard/{{ ceph_mgr.stdout | regex_replace('^-n mgr.(.*)(?P<inst>) -f (.*)+$', '\\1') }}/server_addr \
+    mgr mgr/dashboard/{{ current_mgr }}/server_addr \
     {{ hostvars[dashboard_backend][tripleo_ceph_dashboard_net] }}
   become: true
-  changed_when: false
   vars:
     tripleo_ceph_dashboard_net: "{{ service_net_map['ceph_grafana_network'] + '_ip' }}"