Datashader instance driven by configuration, not user set / saved object captured

[ndmitch311]

Currently when a user creates a Datashader layer it is prepopulated with a URL that is set in configuration , but can be modified by the user. The instance of Datashader used on a Map/Dashboard should always be the one that points to the same Elasticsearch as the Kibana instance because it is within the Datashader configuration that the ES instance utilized is defined, not based on the Kibana used.

ENHANCEMENTS:
-Change Kibana UI so URL is not user editable.
-Change Map Component/Dashboard save so URL is not captured in SavedObjects, but rather read from config when the MapDashboard is utilized.