stable-2.2: Introduce ceph_nfs_ceph_user

In analogy to ceph_nfs_rgw_user, we should be able to define a user
with which the nfs-ganesha Ceph FSAL connects to the cluster.

Introduce a ceph_nfs_ceph_user, setting its default to "admin" (which
preserves the prior behavior of always connecting as client.admin).

Backport of #1911.

roles/ceph-common/templates/ganesha.conf.j2

@@ -20,6 +20,7 @@ EXPORT
 
 	FSAL {
 		Name = CEPH;
+		User_Id = "{{ ceph_nfs_ceph_user }}";
 	}
 }
 {% endif %}

roles/ceph-nfs/defaults/main.yml

@@ -37,6 +37,7 @@ ceph_nfs_ceph_export_id: 20134
 ceph_nfs_ceph_pseudo_path: "/cephobject"
 ceph_nfs_ceph_protocols: "3,4"
 ceph_nfs_ceph_access_type: "RW"
+ceph_nfs_ceph_user: "admin"
 
 ###################
 # FSAL RGW Config #