ceph-volume: allow parallel creates

ceph-volume: allow parallel creates

Currently the create code decides the vg_name "ceph-$cluster_fsid" as
the primary vg_name and creates a new name if this already exists.

If this code is run N times in parallel, the script will try to
create N times the vg with the name "ceph-$cluster_fsid" and it
will fail to create the N osds successfully.

Creating vgs with names like "ceph-$uuid4" lets our scripts to run
without any problems.

Signed-off-by: Theofilos Mouratidis <t.mour@cern.ch>