Support Router Advertisement Daemon (radvd) for IPv6

Install radvd if the l3 service is enabled.

Partially implements blueprint: neutron-ipv6-radvd-ra

Change-Id: I749b31dcaad072a35e5921579b50f1ca451b1383
diff --git a/files/rpms/neutron b/files/rpms/neutron
index 15ed973..f02c02b 100644
--- a/files/rpms/neutron
+++ b/files/rpms/neutron
@@ -22,3 +22,4 @@
 qpid-cpp-server        # NOPRIME
 sqlite
 sudo
+radvd # NOPRIME