pmbootstrap config -h: show keys / tab complete

@unrznbl wrote in !1728 (merged):

Ideally I'd like to see ./pmbootstrap config -h show all options available.

Turns out, it was easy to implement that. Would you like to review this small change? 😃

(Tab completion setup is described here: https://wiki.postmarketos.org/wiki/Installing_pmbootstrap#Tab_completion)

user@pm-devel ~/code/pmbootstrap (git)-[pmaports-in-conf] % pmbootstrap config -h
usage: pmbootstrap config [-h] [name] [value]

positional arguments:
  name        variable name, one of: aports, ccache_size, device,
              extra_packages, hostname, jobs, kernel, keymap,
              nonfree_firmware, nonfree_userland, qemu_native_mesa_driver,
              ssh_keys, timezone, ui, user, work
  value       set variable to value

optional arguments:
  -h, --help  show this help message and exit
user@pm-devel ~/code/pmbootstrap (git)-[pmaports-in-conf] % pmbootstrap config nonfree_
nonfree_firmware  nonfree_userland