ceph: missing dependency on py3-prettytable

ceph, the main utility, depends on prettytable, but it is not installed by default:

alpine:/etc/ceph# ceph -s
Traceback (most recent call last):
  File "/usr/bin/ceph", line 151, in <module>
    from ceph_daemon import admin_socket, DaemonWatcher, Termsize
  File "/usr/lib/python3.8/site-packages/ceph_daemon.py", line 24, in <module>
    from prettytable import PrettyTable, HEADER
ModuleNotFoundError: No module named 'prettytable'
alpine:/etc/ceph# apk add py3-prettytable
(1/1) Installing py3-prettytable (0.7.2-r5)
OK: 357 MiB in 216 packages
alpine:/etc/ceph# ceph -s
[errno 2] error connecting to the cluster
alpine:/etc/ceph#