Bash completion: Small optimizations

$2 doesn't contain the correct value when completing something like
"--disk-template=…". Getting it via COMP_WORDS is better.

Short options (e.g. -I) can't have an equal sign.

Also add useful debugging commands for development.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>