Enable bash completion for htools scripts

This patch ties together the previous commits, by extending
build-bash-completion to call all htools scripts and get their
completion information, then use that to build fake cli_options
representing them and finally generate the bash completion
information.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>