ceph-disk-prepare: use os.path.realpath()

My janky symlink resolution is broken in various ways.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: clean up stupid check for a digit

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: verify device is not mounted before using

Make sure the data and/or journal device(s) are not in use (mounted)
before using them.  Make room for additional "in-use" checks in the future.

Closes: #3256
Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: verify device is not in use by device-mapper

Be nice and tell the user which devices/mappings are consuming the device,
too.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-prepare: move in-use checks to the top, before zap

Move the in-use checks to the very top, before we (say) zap!

Signed-off-by: Sage Weil <sage@inktank.com>
Merge branch 'wip-prepare'

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Alexandre Marangone <alexandre.marangone@inktank.com>
Tested-by: Tamil Muthamizhan <tamil.muthamizhan@inktank.com>