mgr/cephadm: allow osd spec removal

mgr/cephadm: allow osd spec removal

OSD specs/drivegroups are essentially templates for OSD creation but do
not map to the full lifecycle of the OSDs that they create.  When a spec
is removed, remove it immediately.

If no --force is provided, the error lists which OSDs will be left behind.
If --force is passed, the service is removed.

This leaves behind a few oddities:

- When you list services, OSDs that were created by the drivegroup may
  still exist, causing the drivegroup to appear in the list as
  unmanaged services.
- If you create a new drivegroup with the same name, the prior OSDs will
  appear to belong to the new spec instance, regardless of whether the
  spec/drivegroup parameters are the same.

Signed-off-by: Sage Weil <sage@newdream.net>