Cluster Executor permission may only require EXEC

With ISPN-9904 Cluster Executor now requires ADMIN privileges when retrieved. This is due to the nature of operating on the EmbeddedCacheManager in a remote node. We may want to reduce the required privilege to only EXEC, but then we would also have to ensure the Subject is passed along in the remote operation to ensure this privilege is maintained.

See https://github.com/infinispan/infinispan/pull/6606#discussion_r251807567 & https://github.com/infinispan/infinispan/pull/6606#discussion_r251807097 for more discussion.