Feature/filestorage sharenetworks delete (#122)

* sfs: Add delete for share networks

* sfs: Make name and descr required for creating share network

* sfs: Add acceptance test for share network Delete

* sfs: Remove required parameters

After taking a close look at the code it appeared that 'name'
and 'description' are not required parameters
diff --git a/openstack/sharedfilesystems/v2/sharenetworks/urls.go b/openstack/sharedfilesystems/v2/sharenetworks/urls.go
index 637ea98..7cd6c73 100644
--- a/openstack/sharedfilesystems/v2/sharenetworks/urls.go
+++ b/openstack/sharedfilesystems/v2/sharenetworks/urls.go
@@ -5,3 +5,7 @@
 func createURL(c *gophercloud.ServiceClient) string {
 	return c.ServiceURL("share-networks")
 }
+
+func deleteURL(c *gophercloud.ServiceClient, id string) string {
+	return c.ServiceURL("share-networks", id)
+}