Parametrize rndc key algorithm for designate

Change-Id: Ic0458a466f73e7d212a3ca0f8a2a4f3ebf131060
diff --git a/designate/files/rndc.key b/designate/files/rndc.key
index dac4358..400683e 100644
--- a/designate/files/rndc.key
+++ b/designate/files/rndc.key
@@ -1,5 +1,5 @@
 {%- from "designate/map.jinja" import server with context -%}
 key "rndc-key" {
-        algorithm hmac-md5;
+        algorithm "{{ server.backend.bind9.rndc_algorithm }}";
         secret "{{ server.backend.bind9.rndc_key }}";
 };