(POOLER-70) Rename conflicting method in vSphere Provider

Previously, the vSphere Provider had two methods called
`find_least_used_compatible_host`: one in the base class and one in the vSphere
helper methods.  This commit renames the vSphere helper methods and tests to
`find_least_used_vsphere_compatible_host` to stop the conflict.

lib/vmpooler/providers/vsphere.rb

@@ -265,7 +265,7 @@ module Vmpooler
           cluster_hosts
         end
 
-        def find_least_used_compatible_host(vm)
+        def find_least_used_vpshere_compatible_host(vm)
           ensure_connected @connection, @credentials
 
           source_host = vm.summary.runtime.host