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/apts/neutron b/files/apts/neutron
index 648716a..92e0a06 100644
--- a/files/apts/neutron
+++ b/files/apts/neutron
@@ -23,3 +23,4 @@
 qpidd # NOPRIME
 sqlite3
 vlan
+radvd # NOPRIME