Rename nvp_cluster for VMware NSX plugin

Rename the module into nsx_cluster and the class into NSXCluster,
and update references accordingly.

Also, rename utility method create_nvp_cluster to create_nsx_cluster
and move it into nsx_utils.

Partial-implements blueprint: nicira-plugin-renaming

Change-Id: I1a943cc64730ea67f464c3804370dad46411f8a2
Merge "Rename nvp_cluster for VMware NSX plugin"