Rename `ClusterPlatformConfigureWorker` to `ClusterConfigureWorker`

This is a follow up of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22308.

After https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22308, we'll have two workers that call Clusters::RefreshService to create missing kubernetes namespaces for specific Cluster or for clusters in a Projects:

• ClusterPlatformConfigureWorker
• ClusterProjectConfigureWorker.

I'd vouch to rename ClusterPlatformConfigureWorker to simply ClusterConfigureWorker