Enhance import_dashboards script to load the dashboards via Kibana API

[monicasarbu]

There are two use cases where loading the dashboards from Beats 5.x fails:

1. when loading the dashboards to Elasticsearch/Kibana 6.0
2. after the user presses the button "reindexing" in Kibana 5.6

Both use cases can be fixed by adding the option in Beats 5.6 to load the dashboards via the Kibana API.

A solution would be to add an option to the import_dashboards script available in Beats 5.6 to load the dashboards via Kibana API, only when the user requested that explicitly. By default, Beats 5.6 are loading the dashboards to Elasticsearch.

cc-ed @tylersmalley @simianhacker

[monicasarbu]

It's implemented in PR #4682