cephadm: improve cephadm pull usage message

cephadm: improve cephadm pull usage message

Generally, it's uncommon for users to run this
directly, but in case they need to for debugging
purposes, we should include how to pass the
image to be pulled in the usage message.

Additionally, include that this is only to be used
for pulling ceph images in the help message, as
that isn't necessarily clear. Pulling anything
else will result in a traceback as it tries
to run `ceph --version` inside the container.

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