Optimize kibana caching and bundles at build time

Previously when spinning kibana docker container a log message warned
of kibana having to optimize and caching bundles, as such:

{
 "type":"log",
 "@timestamp":"2018-10-18T15:31:44Z",
 "tags":["info","optimize"],
 "pid":1,
 "message":"Optimizing and caching bundles...This may take few minutes"
}

This would cause kibana to be unreachable for few minutes when starting
up the analytics dashboard.

This fix moves the optimization at image building time rather than at
run time, as suggested by:

elastic/kibana#6057 (comment)

Feature: Issue 9870

Change-Id: I837d612c988aeebe2d5911003a82cd43e65c6d4b

kibana/Dockerfile

@@ -1,2 +1,3 @@
 FROM docker.elastic.co/kibana/kibana:5.5.2
 RUN kibana-plugin remove x-pack
+RUN /usr/local/bin/kibana-docker 2>&1 | grep -m 1 "Optimization of .* complete"