Pin latest ceilometer packages from mos9.0-proposed

Repository 9.0-latest contains last 9.2 bug fixes for Ceilometer Mitaka.
These updates fix ElasticSearch requests from Event API.
Packages for MOS 9.0 have broken ES Event API.

Also, it contains human readable errors for ES resource backends
if resource connection is not defined.
diff --git a/linux/system/repo/mos9_latest.yml b/linux/system/repo/mos9_latest.yml
index a55c8ae..d986602 100644
--- a/linux/system/repo/mos9_latest.yml
+++ b/linux/system/repo/mos9_latest.yml
@@ -5,14 +5,20 @@
         python-msgpack:
           version: latest
       repo:
-      # This repository is needed because the python-influxdb package
-      # required for Mitaka Ceilometer is only present in
-      # mos9.0-proposed from the 9.0-latest repository
         mirantis_latest_openstack_proposed:
           source: "deb http://mirror.fuel-infra.org/mos-repos/ubuntu/snapshots/9.0-latest/ mos9.0-proposed main"
           architectures: amd64
           key_url: "http://mirror.fuel-infra.org/mos-repos/ubuntu/snapshots/9.0-latest/archive-mos9.0.key"
           pin:
+            # Ceilometer packages should be picked from mos9.0-proposed
+            # if more recent versions exist there because the packages
+            # from mos9.0-stable have bugs related to the Events API.
+            - pin: "release a=mos9.0-proposed"
+              priority: 800
+              package: "/ceilometer/"
+            # This repository is needed because the python-influxdb package
+            # required for Mitaka Ceilometer is only present in
+            # mos9.0-proposed from the 9.0-latest repository
             - pin: "release a=mos9.0-proposed"
               priority: 400
               package: "*"