Add support for signing artifacts with sigul

Signing is disabled by default as it appears the current sigul
implementation cannot handle the amount of artifacts being pushed
to it at this time. Turn it off in the meantime until we can get
that working.

Issue: https://jira.linuxfoundation.org/browse/RELENG-1614
Change-Id: I8823884ca8b9fc600c4a31b1af94f4b95101f414
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>