cephadm: make custom_configs work for tcmu-runner container

cephadm: make custom_configs work for tcmu-runner container

This is intended to be a temporary workaround to make
custom config files be able to be mounted into
the tcmu-runner container. The hope is to refactor
cephadm's iscsi handling for squid, but a patch
like this could be useful for iscsi in older
releases where currently custom config files
are unusable for the tcmu-runner container

What this patch actually does is have us write the
custom config files to a dir for the tcmu-runner
container so that the rest of the logic works without
change. I thought this would be easier to remove later
than a patch that integrates more with the container
mounts or general deployment

The use case in mind is something like

service_type: iscsi
service_id: foo
service_name: iscsi.foo
placement:
  hosts:
  - host1
custom_configs:
  -  mount_path: /etc/tcmu/tcmu.conf
     content: |
       log_level = 4
spec:
  api_password: admin
  api_port: 5000
  api_user: admin
  pool: foo

which would allow users to modify the logging of the
tcmu-runner container for debugging purposes

Signed-off-by: Adam King <adking@redhat.com>