ceph-disk: compatibility fix for python 3

ceph-disk: compatibility fix for python 3

In python 3, dev is a string, but mounts_dev is bytes (because
/proc/mounts was opened with mode 'rb') so they can't compare
equal, resulting in is_mounted() returning None for mounted OSDs.
The safest fix for this we could come up with was to normalize
dev to a str using _bytes2str() (just in case), and open
/proc/mounts in mode 'r', so its lines are interpreted as strs.

Fixes: https://tracker.ceph.com/issues/35906
Signed-off-by: Tim Serong <tserong@suse.com>