ceph-disk: work around buggy rhel/centos parted

parted on RHEL/Centos prefixes the *machine readable output* with

 1b 5b 3f 31 30 33 34 68

Note that the same thing happens when you 'import readline' in python.

Work around it!

Signed-off-by: Sage Weil <sage@inktank.com>