utils: Add function to find items in dictionary using regex

This basically extracts a small piece of code from ganeti-rapi and puts
it into a utility function. RAPI resources are found using a dictionary
in which the keys can either be static strings or compiled regular
expressions. This might be handy in other places, hence extracting it
and adding unittests.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>