Move innards of cluster method to environment
Move innards of cluster method to environment

Environment already has a memoized deployment plaform.

No need to check for respond_to?(:cluster) now that all deployment
platforms are Clusters::Cluster.

stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq