Add support for "scoped" autohold requests, making it possible
to create requests that match a given ref filter and hold nodes
for build that matches the filter.
Introduce two new arguments to `zuul autohold`: --ref and --change,
with --ref accepting regex that builds are matched against, and
--change being a shortcut that creates a filter for the specific change.
Change-Id: I801ba1af4b1bda46abff07791dd96828f2738621
Krzysztof Klimonda
37d5403629