cephadm: ceph-iscsi first draft

cephadm: ceph-iscsi first draft

This if the first draft of the ceph-iscsi in cephadm.
There are a few gotchas when running `rbd-target-api` in a container:

 1. We need both the ceph.conf and iscsi-gateway.cfg, so needed to
ability to pass extra config. This latter is based off the spec, so now
the daemon config func api allows you to return a dict of configs:

  { 'config': '<str>' # will be appended to the ceph.conf
    '<conf name>': 'str', # Will be dumped in datadir/<conf name>
    ...
  }

It will be up to cephadm to know to bind mount it to the right location.
The first 'config' isn't used by this patch, but makes it possible for
specs or config funcs to append anything? maybe it's overkill.

 2. We need the kernel's configfs in the container so we can configure
LIO. There is a chicken and egg problem, configfs isn't mounted on the
host to bind mount when the container starts. So now a check is added to
the `unit.run` and cleanup in the `unit.poststop` scripts for
daemon_type iscsi.

 3. rbd-target-api is python and hardcodes a few things, like logging
through `/dev/log` which happens to be a domain socket. So `/dev/log`
also needed to be bind mounted into the continer.

 4. The daemon expects the keyring to be in `/etc/ceph` so this needed to
be specifically bind mounted to the correct location too.

As this currently stands this is deploying and starting the api on port
5000, so seems to be "working", also gateway.conf does exist in the
pool. I have yet to set up an iscsi device, but will test that next.

The `rbd-target-api` daemon expects the ssl key and cert to be named a
certain name in the contianer. So SSL isn't working yet. However, I do
hav a PR in ceph-iscsi to look in the mon config-key store for them[0].

[0] - https://github.com/ceph/ceph-iscsi/pull/173

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