ceph-volume: `get_first_*()` refactor

ceph-volume: `get_first_*()` refactor

As indicated by commit 17957d9beb42a04b8f180ccb7ba07d43179a41d3 those
fuctions were meant to avoid writing something like following:

```
lvs = get_lvs()
if len(lvs) >= 1:
	lvs = lv[0]
```

Those functions should return `None` if 0 or more than 1 item is returned.
The current name of these functions are confusing and can lead to thinking that
we just want the first item returned, even though it returns more than 1
item, let's rename them to `get_single_pv()`, `get_single_vg()` and
`get_single_lv()`

Closes: https://tracker.ceph.com/issues/49643
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>