Set grafana-server user and password in ceph-dashboard role

Set grafana-server user and password in ceph-dashboard role

This change adds two tasks to set grafana-api user and password
that are required to inject dashboard layouts to the external
grafana instance.
Without these two parameters the ceph-ansible playbook fails
showing an authorization error (HTTPError: 401 Client Error:
Unauthorized").

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1767365
Signed-off-by: fmount <fpantano@redhat.com>
(cherry picked from commit 41b8c17356fa1273761c3d864f959fbcb11813e7)