Make ceph-nfs configurable

Ceph-nfs image is not available for Debian build. Because there is
no Debian package under official ceph-nfs' repo:
http://download.ceph.com/nfs-ganesha/deb-V2.5-stable/luminous/dists/

Change-Id: I35d84296318a849616e72fb6a2064b8ab62fdaf7

ansible/roles/ceph/tasks/deploy.yml

@@ -13,7 +13,9 @@
   when: inventory_hostname in groups['ceph-mgr']
 
 - include: start_nfss.yml
-  when: inventory_hostname in groups['ceph-nfs']
+  when:
+    - enable_ceph_nfs | bool
+    - inventory_hostname in groups['ceph-nfs']
 
 - include: bootstrap_osds.yml
   when: inventory_hostname in groups['ceph-osd']

ansible/roles/ceph/tasks/pull.yml

@@ -43,4 +43,5 @@
     common_options: "{{ docker_common_options }}"
     image: "{{ ceph_nfs_image_full }}"
   when:
+    - enable_ceph_nfs | bool
     - inventory_hostname in groups['ceph-nfs']