jenkins: add in optional plugins

Let's take the opportunity of this next upgrade to
add in the plugins for slack and prometheus.  At
some point in the future prometheus may be used to
monitor Jenkins, and slack gives us another notification
option.

Signed-off-by: Kelley Spoon <kelley.spoon@linaro.org>
Change-Id: Ifb0b3d33a70cd315ff4f1d6cf6df481047e15ac8
diff --git a/jessie-amd64-jenkins-master/plugins.txt b/jessie-amd64-jenkins-master/plugins.txt
index 99dd282..83e4a2b 100644
--- a/jessie-amd64-jenkins-master/plugins.txt
+++ b/jessie-amd64-jenkins-master/plugins.txt
@@ -29,11 +29,13 @@
 pipeline-github-lib:36.v4c01db_ca_ed16
 pipeline-githubnotify-step:49.vf37bf92d2bc8
 pipeline-utility-steps:2.12.0
+prometheus:2.0.11
 postbuildscript:3.1.0-375.v3db_cd92485e1
 rebuild:1.33.1
 role-strategy:3.2.0
 ssh-agent:1.24.1
 ssh-slaves:1.806.v2253cedd3295
+slack:608.v19e3b_44b_b_9ff
 timestamper:1.17
 throttle-concurrents:2.7
 workflow-aggregator:2.7