purge_cluster: fix dmcrypt purge

purge_cluster: fix dmcrypt purge

dmcrypt devices aren't closed properly, therefore, it may fail when
trying to redeploy after a purge.

Typical errors:

```
ceph-disk: Cannot discover filesystem type: device /dev/sdb1: Command
'/sbin/blkid' returned non-zero exit status 2
```

```
ceph-disk: Error: unable to read dm-crypt key:
/var/lib/ceph/osd-lockbox/c6e01af1-ed8c-4d40-8be7-7fc0b4e104cf:
/etc/ceph/dmcrypt-keys/c6e01af1-ed8c-4d40-8be7-7fc0b4e104cf.luks.key
```

Closing properly dmcrypt devices allows to redeploy without error.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1492242
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>