use partx for red hat or centos instead of partprobe

Signed-off-by: Alfredo Deza <alfredo@deza.pe>
alert the user about error messages from partx

Signed-off-by: Alfredo Deza <alfredo@deza.pe>
Merge pull request #1205 from ceph/wip-7334

use `partx` for CentOS/RHEL instead of `partprobe`
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>