ceph-disk: run the right executables from udev

When run by the udev rules, PATH is not defined. Thus,
ceph-disk-activate relies on its which() function to locate the
correct executable.  The which() function used os.defpath if none was
set, and this worked for anything using it.

ad6b4b4b08b6ef7ae8086f2be3a9ef521adaa88c added a new default value to
PATH, so only /usr/bin was checked by callers that did not use
which(). This resulted in the mount command not being found when
ceph-disk-activate was run by udev, and thus osds failing to start
after being prepared by ceph-deploy.

Make ceph-disk consistently use the existing helpers (command() and
command_check_call()) that use which(), so lack of PATH does not
matter. Simplify _check_output() to use command(),
another wrapper around subprocess.Popen.

Fixes: #7258
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1156 from ceph/wip-ceph-disk

ceph-disk: run the right executables from udev