background_exec: run something in the background
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 <6dd34506bbd5e58221bbb3e4732d97c91f02277b@inktank.com>

ktdreyer/teuthology,t-miyamae/teuthology,ceph/teuthology,dmick/teuthology,SUSE/teuthology,robbat2/teuthology,ktdreyer/teuthology,zhouyuan/teuthology,robbat2/teuthology,yghannam/teuthology,SUSE/teuthology,yghannam/teuthology,michaelsevilla/teuthology,ivotron/teuthology,caibo2014/teuthology,tchaikov/teuthology,zhouyuan/teuthology,dreamhost/teuthology,dmick/teuthology,SUSE/teuthology,dmick/teuthology,ivotron/teuthology,caibo2014/teuthology,michaelsevilla/teuthology,ceph/teuthology,tchaikov/teuthology,dreamhost/teuthology,t-miyamae/teuthology