pmb.config: use ssh_key_glob for ask_for_ssh_keys (MR 2251)

Since a user can modify the glob to be somewhat arbitrary, use that to
check if any ssh keys exist and ask in pmbootstrap init.

Otherwise ssh_key_glob would only work if it was a subset of
~/.ssh/id_*.pub, so e.g. ~/.ssh/id_foo.pub but ~/.ssh/foo.pub wouldn't
work.