Remove mandatory remote_cluster_client on voting_only master nodes for cross cluster search

[Sp3ci3s8472]

I would like to propose that voting_only masters should not be forced to have the remote_cluster_client role if you want to implement cross cluster search on a cluster. Currently a voting_only master must have that role in order for all search requests to be correctly handled; if the role is not set a client (e.g. Kibana) gets an error if the request lands on that specific master through the load balancer.

Enabling this role can have an averse affect on environments that don't have a lot of cpu/memory resources (e.g. victim sites).

In the following setup, only the master_data nodes should get the remote_cluster_client.
2x master_data
1x voting_master

I'm not familiar enough with the eck stack, but with some pointers I might be able to implement this myself depending on the difficulty.