daemon-helper: optional kill an entire process group

Signed-off-by: Sage Weil <sage@inktank.com>
background_exec: run something in the background

This is a contextmanager task that will run some command in the background
for the duration of any subsequent tasks, and kill it in the cleanup
phase.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #233 from ceph/wip-background

run something in the background