qa/tasks: add a new cephadm_from_container feature to cephadm task

qa/tasks: add a new cephadm_from_container feature to cephadm task

The cephadm_from_container allows one to do a single container build
and then point teuthology at that image as the "single source of truth".
I find this extremely convenient when running teuthology locally and
I keep carrying this patch around - I figure having it upstream will
simplify my workflow. Maybe someday it'll benefit others too.

To use it I set up a yaml overrides file with the following content:
```yaml
overrides:
  cephadm:
    image: "quay.io/phlogistonjohn/ceph:dev"
    cephadm_from_container: true
  verify_ceph_hash: false
verify_ceph_hash: false
```

This let's me test my custom builds fairly easily!

Signed-off-by: John Mulligan <phlogistonjohn@asynchrono.us>