CBD-1217:  add jenkinsReports::last_commit_valid() to show status of last done commit_validation job

Change-Id: I55f9241194a41ebb83e7624ac63ce2cd82768d49
Reviewed-on: http://review.couchbase.org/33338
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>