Description of problem:

For disconnected/offline installations when using Satellite the
official way to set custom images is documented to run ceph set mgr
after bootstrapping the Cluster. Until done, the Cluster is in state
"HEALTH_WARN".

Automated deployment and testing require the additional shell commands
to be executed which makes it difficult and cumber sum.
The Request for Enhancement provides the functionality to bootstrap
with custom images when appropriate Environment variables are set.

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

as recommended by PM to be evaluated for 5.2


Actual results:
manual executing "ceph config set mgr" commands are necessary.


Expected results:
cephadm setting the custom image references during bootstrap


Additional info:
Documentation should note the possibility of setting the images through environment variables.

```
# example usage
$ export DEFAULT_NODE_EXPORTER_IMAGE=quay.io/prometheus/node-exporter:latest
$ export DEFAULT_GRAFANA_IMAGE=private-registry.local/grafana/customer-build:special-plugins

$ cephadm bootstrap ...
```