Skip tripleo_lvmfilter unless enabled or dry_run is set

This is to address the concerns raised in [1]; it skips the role
execution unless the feature is either explicitly enabled or the
caller has set dry_run variable which will make it run up to the
creation of the new lvm.conf file but not actually copy it into
place.

1. https://review.opendev.org/#/c/697990/49/tripleo_ansible/roles/tripleo_lvmfilter/tasks/main.yml

Change-Id: I1c914feda3c4b1bfb6dcfd981244477f82915b75

tripleo_ansible/roles/tripleo_lvmfilter/defaults/main.yml

@@ -19,5 +19,6 @@
 
 # All variables within this role should have a prefix of "tripleo_tripleo_lvmfilter"
 tripleo_tripleo_lvmfilter_enabled: false
+tripleo_tripleo_lvmfilter_dry_run: false
 tripleo_tripleo_lvmfilter_devices_allowlist: []
 tripleo_tripleo_lvmfilter_devices_denylist: []

tripleo_ansible/roles/tripleo_lvmfilter/tasks/main.yml

@@ -19,7 +19,9 @@
   package_facts:
     manager: auto
 - name: gather allowed block devices list
-  when: "'lvm2' in ansible_facts.packages"
+  when:
+    - "'lvm2' in ansible_facts.packages"
+    - tripleo_tripleo_lvmfilter_enabled or tripleo_tripleo_lvmfilter_dry_run
   block:
     - name: collect in-use lvm2 devices list
       become: true