[nfs_mount] Maybe access points don't need to mount any directory

Signed-off-by: Yohann D'ANELLO <ynerant@crans.org>

plays/nfs_mount.yml

 #!/usr/bin/env ansible-playbook
 ---
 # Deploy nfs_mount server
-- hosts: all
+- hosts: server
   vars:
     nfs_mount: "{{ glob_nfs_mount | default({}) | combine(loc_nfs_mount | default({})) }}"
   roles: