ceph-volume: api/lvm create or reuse a vg

ceph-volume: api/lvm create or reuse a vg

This changes create_lv so one can pass the desired device and either a
VG with a name starting with ceph is re-used or a new one is created.
This commit also adds two new lvm primitives, making use of lvm's select
feature. The goal is to eventually avoid keeping a full list of lv's (or
vg's) around and query the lvm system as needed.

Signed-off-by: Jan Fajerski <jfajerski@suse.com>