Rename ServiceClient to ProviderClient.

Rather than store state within service-level Clients, we really want that
at the provider level. Service-level clients should be stateless.
diff --git a/openstack/identity/v3/tokens/urls.go b/openstack/identity/v3/tokens/urls.go
index 5b47c02..ac5a19f 100644
--- a/openstack/identity/v3/tokens/urls.go
+++ b/openstack/identity/v3/tokens/urls.go
@@ -2,6 +2,6 @@
 
 import "github.com/rackspace/gophercloud"
 
-func getTokenURL(c *gophercloud.ServiceClient) string {
+func getTokenURL(c *gophercloud.ProviderClient) string {
 	return c.ServiceURL("auth", "tokens")
 }