Change plugin id for remote_clusters to remoteClusters

[joshdover]

The Platform now logs a warning when plugin IDs are not in camelCase format. remote_clusters needs to be renamed to remoteClusters in order to make this warning go away. This is important because this warning is confusing to users. We could just log the message in development, but it seems just as easy to just go ahead and do the rename now.

All this should entail is changing the id field in the kibana.json file and updating any plugins that depend on this plugin to use the new id when calling this plugin's APIs.

The config path does not need to change and should remain in snake_case.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)