Description of problem:
the site-docker playbook fails in [1] because ceph_nfs_ceph_user is set to a string, not a dict

there seem to be multiple references in the code to the variable which sometimes is assumed to be a string [1] but others a dict [2]

probably related to [3]

1. https://github.com/ceph/ceph-ansible/blob/master/roles/ceph-nfs/tasks/main.yml#L29
2. https://github.com/ceph/ceph-ansible/blob/master/group_vars/clients.yml.sample#L60
3. https://github.com/ceph/ceph-ansible/commit/6a6785b719d126cf54ebad8b2a22c97d90afd05e

Version-Release number of selected component (if applicable):
4.0.14

How reproducible:
deploy ceph-nfs and client connecting to an external ceph cluster