ceph-nfs: fix ceph_nfs_ceph_user variable
ceph-nfs: fix ceph_nfs_ceph_user variable

The ceph_nfs_ceph_user variable is a string for the ceph-nfs role but a
list in ceph-client role.
6a6785b introduced a confusion between both variable type in the ceph-nfs
role for external ceph with ganesha.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1801319

Signed-off-by: Dimitri Savineau <a50a620e679eed14f4e99998b821cea8884cf228@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible