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/requests.go b/openstack/sharedfilesystems/v2/sharenetworks/requests.go
index f551861..ad29404 100644
--- a/openstack/sharedfilesystems/v2/sharenetworks/requests.go
+++ b/openstack/sharedfilesystems/v2/sharenetworks/requests.go
@@ -44,3 +44,9 @@
 	})
 	return
 }
+
+// Delete will delete the existing ShareNetwork with the provided ID.
+func Delete(client *gophercloud.ServiceClient, id string) (r DeleteResult) {
+	_, r.Err = client.Delete(deleteURL(client, id), nil)
+	return
+}