Feature/filestorage securityservices list (#134)

* sfs: Add support for security services List

* sfs: Add acceptance tests for security service List

* sfs: Remove extra parameters for security service list

After taking a closer look at the code, some parameters
seem to be filtered out during the list request. They
have now been filtered out.

* sfs: Fix unit tests

* sfs: Use SecurityServiceType for ListOpts Type
diff --git a/openstack/sharedfilesystems/v2/securityservices/urls.go b/openstack/sharedfilesystems/v2/securityservices/urls.go
index 5cbdd17..e4deb5f 100644
--- a/openstack/sharedfilesystems/v2/securityservices/urls.go
+++ b/openstack/sharedfilesystems/v2/securityservices/urls.go
@@ -9,3 +9,7 @@
 func deleteURL(c *gophercloud.ServiceClient, id string) string {
 	return c.ServiceURL("security-services", id)
 }
+
+func listURL(c *gophercloud.ServiceClient) string {
+	return c.ServiceURL("security-services", "detail")
+}