ceph-disk: improve base device/partition number calculation

ceph-disk: improve base device/partition number calculation

split_dev_base_partnum fails to handle the device nodes names containing
digits (like those /dev/disk/by-id generated by udev), as a result

ceph-deploy osd prepare $node:/dev/sdc3:/dev/disk/by-id/wwn-0x50014ee00386a548-part3

fails, although it works fine this way

ceph-deploy osd prepare $node:/dev/sdc3:/dev/sde3

where /dev/sde3 is the very same partition (i.e. it's the destination of
the /dev/disk/by-id/wwn-0x50014ee00386a548-part3 symlink).

The problem has been silently fixed in the master branch by commits

https://github.com/ceph/ceph/commit/0e34742b968e
https://github.com/ceph/ceph/commit/3bc95dfc1b88
https://github.com/ceph/ceph/commit/77ff7c3dc6dd

Cherry pick those, and https://github.com/ceph/ceph/commit/1c5fea67
which fixes yet another symlink related problem.

Closes-Bug: #1529841
Change-Id: I37f83266efc4468103afd3fe82336b83f91fbe58