inventory_util.py: Make get_cluster() and get_cluster_variable() non-static
inventory_util.py: Make get_cluster() and get_cluster_variable() non-static

Dramatically improves performance by allowing the InventoryUtil's
MultiInventory instance to be shared with Cluster instances, instead
of forcing new Cluster instances to load their own MultiInventory.

drewandersonnz/openshift-tools,ivanhorvath/openshift-tools,drewandersonnz/openshift-tools,openshift/openshift-tools,ivanhorvath/openshift-tools,ivanhorvath/openshift-tools,blrm/openshift-tools,blrm/openshift-tools,drewandersonnz/openshift-tools,openshift/openshift-tools,drewandersonnz/openshift-tools,openshift/openshift-tools,openshift/openshift-tools,openshift/openshift-tools,drewandersonnz/openshift-tools,ivanhorvath/openshift-tools,openshift/openshift-tools,ivanhorvath/openshift-tools,drewandersonnz/openshift-tools,blrm/openshift-tools,blrm/openshift-tools,ivanhorvath/openshift-tools,blrm/openshift-tools,blrm/openshift-tools