ceph-defaults: pin prometheus container tags

ceph-defaults: pin prometheus container tags

In addition to the grafana container tag change, we need to do the same
for the prometheus container stack based on the release present in the
OSE 4.1 container image.

$ docker run --rm openshift4/ose-prometheus-node-exporter:v4.1 --version
node_exporter, version 0.17.0
  build user:       root@67fee13ed48f
  build date:       20191023-14:38:12
  go version:       go1.11.13
$ docker run --rm openshift4/ose-prometheus-alertmanager:4.1 --version
alertmanager, version 0.16.2
  build user:       root@70b79a3f29b6
  build date:       20191023-14:57:30
  go version:       go1.11.13
$ docker run --rm openshift4/ose-prometheus:4.1 --version
prometheus, version 2.7.2
  build user:       root@12da054778a3
  build date:       20191023-14:39:36
  go version:       go1.11.13

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