ceph-volume: quick fix in zap.py

ceph-volume: quick fix in zap.py

`api.get_single_pv(filters={'lv_uuid': lv.lv_uuid})` needs to be called
only if `--destroy` is passed in order to remove vg and pv when there's
nothing left.

With old deployments, it is possible that a lv_uuid matches more than 1
PV.
Given that `get_single_pv()` is only needed when `--destroy` is passed,
let's move this call where it is actually needed.

This makes `ceph-volume lvm zap` fail even though

Fixes: https://tracker.ceph.com/issues/59210
Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>