web: Added cockpit.util.parse_words and cockpit.util.quote_words.

Reviewed-by: Stef Walter <stefw@redhat.com>
web: Properly quote/parse container commands.

By using the new cockpit_quote_words and cockpit_parse_words
functions, and also by avoiding the container.Command property
reported by Docker.

Fixes #451
Fixes #280
Closes #633

Reviewed-by: Stef Walter <stefw@redhat.com>