ceph-volume: fix lvm migrate without args

ceph-volume: fix lvm migrate without args

When running the `lvm migrate` subcommand without any args then the
ceph-volume command fails with a stack trace.

Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/ceph_volume/decorators.py", line 59, in newfunc
    return f(*a, **kw)
  File "/usr/lib/python3.6/site-packages/ceph_volume/main.py", line 151, in main
    terminal.dispatch(self.mapper, subcommand_args)
  File "/usr/lib/python3.6/site-packages/ceph_volume/terminal.py", line 194, in dispatch
    instance.main()
  File "/usr/lib/python3.6/site-packages/ceph_volume/devices/lvm/main.py", line 46, in main
    terminal.dispatch(self.mapper, self.argv)
  File "/usr/lib/python3.6/site-packages/ceph_volume/terminal.py", line 194, in dispatch
    instance.main()
  File "/usr/lib/python3.6/site-packages/ceph_volume/devices/lvm/migrate.py", line 520, in main
    self.migrate_osd()
  File "/usr/lib/python3.6/site-packages/ceph_volume/decorators.py", line 16, in is_root
    return func(*a, **kw)
  File "/usr/lib/python3.6/site-packages/ceph_volume/devices/lvm/migrate.py", line 403, in migrate_osd
    if self.args.osd_id:
AttributeError: 'Migrate' object has no attribute 'args'

That's because we're exiting the parse_argv function but we continue to
execute the migrate_osd function. We should instead exit from the main function.

This update the parsing argument to have the same code than new-db and
new-wal classes.
Now the parsing is done in the make_parser function but the argv testing is
done in the main function allowing to exit the program and displaying the
help message when no arguments are provided.

Fixes: https://tracker.ceph.com/issues/51811
Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>