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>