Use anchors on JJB verify / merge job definitions

We need a relatively easy way of keeping our Gerrit and GitHub verify
and merge jobs in sync. Given that presently the only difference really
should be the SCM and Trigger diffinitions for each of the job types,
plus a bunch of shared boiler plate in document anchors make a good
method for dealing with it.

Issue: RELENG-106
Change-Id: I28dba9cd5e5535353d9bd4214e38fc4f8e34fc07
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>