IPv6 ND Router discovery data plane (VPP-1095)

Add API call to send Router Solicitation messages.
Save info from incoming Router Advertisement messages and notify listeners.

Change-Id: Ie518b5492231e03291bd4c4280be4727bfecab46
Signed-off-by: Juraj Sloboda <jsloboda@cisco.com>

test/util.py

@@ -65,6 +65,11 @@ def mk_ll_addr(mac):
     return addr
 
 
+def ip6_normalize(ip6):
+    return socket.inet_ntop(socket.AF_INET6,
+                            socket.inet_pton(socket.AF_INET6, ip6))
+
+
 class NumericConstant(object):
     __metaclass__ = ABCMeta