Change grafana plugin install logic

Changing the way in which we specify grafana plugins to install. This is
due to a change upstream which now allows plugins to be installed in an
additive method instead of replacing the upstream default plugins [1].

[1] https://review.opendev.org/c/openstack/kolla/+/913184

Co-authored-by: Will Szumski <will@stackhpc.com>

Author: technowhizz

etc/kayobe/kolla.yml

@@ -360,6 +360,9 @@ kolla_build_blocks:
 kolla_build_customizations_common:
   bifrost_base_pip_packages_append:
     - /additions/*
+  grafana_plugins_append:
+    - grafana-piechart-panel
+    - vonage-status-panel
   ironic_inspector_pip_packages_append:
     - /additions/*
   neutron_server_packages_append: