cephadm: Make ceph-iscsi SSL aware

cephadm: Make ceph-iscsi SSL aware

Ceph-iscsi's `rbd-target-api.py` supports listening over SSL if you
provide an SSL cert and key. Originally the script is opinionated and
requires these files to be named `/etc/ceph/iscsi-gateway.{crt,key}`.

When dealing with containers, having to place files inside a container to
enable SSL isn't very clean. To make things easier, like RGW, you can
now place the SSL cert and key data in the mon config-key store.

This will mean there are 2 ways to enable SSL in ceph-iscsi via orch/cephadm.

1. Push the SSL key and cert into the mon config-key under the keys, and
   then make sure api_secure is enabled (requires json):

  iscsi/{clientname}/iscsi-gateway.crt
  iscsi/{clientname}/iscsi-gateway.key

2. Provide the SSL key and cert in the json you pass the orchestrator and
   it'll push them up for you.

Also lockdown the caps so the container can only access iscsi ssl
key/certs.

Signed-off-by: Matthew Oliver <moliver@suse.com>