crimson/admin: add support for 'config help'

crimson/admin: add support for 'config help'

This commit solves a problem unveiled in teuthology testing
where `ceph --admin-daemon ... config help` were constantly
returning `EINVAL`:

```
2021-03-03T02:43:01.566 DEBUG:tasks.admin_socket:Testing config help with config {'branch': 'master'}
2021-03-03T02:43:01.567 DEBUG:teuthology.orchestra.run.smithi083:> sudo adjust-ulimits ceph-coverage /home/ubuntu/cephtest/archive/coverage ceph --admin-daemon /var/run/ceph/ceph-osd.0.asok config help
2021-03-03T02:43:01.711 INFO:teuthology.orchestra.run.smithi083.stderr:no valid command found; 3 closest matches:
2021-03-03T02:43:01.712 INFO:teuthology.orchestra.run.smithi083.stderr:config get <var>
2021-03-03T02:43:01.712 INFO:teuthology.orchestra.run.smithi083.stderr:config set <var> <val>...
2021-03-03T02:43:01.712 INFO:teuthology.orchestra.run.smithi083.stderr:config show
2021-03-03T02:43:01.713 INFO:teuthology.orchestra.run.smithi083.stderr:admin_socket: invalid command
2021-03-03T02:43:01.714 DEBUG:teuthology.orchestra.run:got remote process result: 22
2021-03-03T02:43:01.714 INFO:tasks.admin_socket:ceph cli returned an error, command not registered yet?
2021-03-03T02:43:01.715 INFO:tasks.admin_socket:sleeping and retrying ...
```

The root cause is no support for `config help` in crimson.
As shown below this command hasn't been registered at all:

```
DEBUG 2021-03-03 02:42:48,242 [shard 0] osd - start: asok socket path=/var/run/ceph/ceph-osd.0.asok
INFO  2021-03-03 02:42:48,242 [shard 0] osd - register_command(): mark_unfound_lost)
INFO  2021-03-03 02:42:48,242 [shard 0] osd - register_command(): query)
INFO  2021-03-03 02:42:48,242 [shard 0] osd - register_command(): perf dump_seastar)
INFO  2021-03-03 02:42:48,242 [shard 0] osd - register_command(): dump_pgstate_history)
INFO  2021-03-03 02:42:48,242 [shard 0] osd - register_command(): flush_pg_stats)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): send_beacon)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): status)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): injectargs)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): config show)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): config set)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): config get)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): get_command_descriptions)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): help)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): git_version)
INFO  2021-03-03 02:42:48,243 [shard 0] osd - register_command(): version)
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>