Add metricbeat kafka dashboard (elastic#8457)

Add metricbeat kafka dashboard based on the one implemented by
Dale for demo.elastic.co.

Co-authored-by: Dale McDiarmid <dalem@elastic.co>

CHANGELOG.asciidoc

@@ -87,6 +87,7 @@ https://github.com/elastic/beats/compare/v6.4.0...master[Check the HEAD diff]
 - Added io disk read and write times to system module {issue}8473[8473] {pull}8508[8508]
 - Avoid mapping issues in kubernetes module. {pull}8487[8487]
 - Fix issue that would prevent kafka module to find a proper broker when port is not set {pull}8613[8613]
+- Add Kafka dashboard. {pull}8457[8457]
 
 *Packetbeat*
 

metricbeat/docs/modules_list.asciidoc

@@ -62,7 +62,7 @@ This file is generated! See scripts/docs_collector.py
 |<<metricbeat-metricset-http-server,server>> beta[]  
 |<<metricbeat-module-jolokia,Jolokia>>     |image:./images/icon-no.png[No prebuilt dashboards]    |  
 .1+| .1+|  |<<metricbeat-metricset-jolokia-jmx,jmx>>   
-|<<metricbeat-module-kafka,Kafka>>  beta[]   |image:./images/icon-no.png[No prebuilt dashboards]    |  
+|<<metricbeat-module-kafka,Kafka>>  beta[]   |image:./images/icon-yes.png[Prebuilt dashboards are available]    |  
 .2+| .2+|  |<<metricbeat-metricset-kafka-consumergroup,consumergroup>> beta[]  
 |<<metricbeat-metricset-kafka-partition,partition>> beta[]  
 |<<metricbeat-module-kibana,Kibana>>  beta[]   |image:./images/icon-no.png[No prebuilt dashboards]    |