FIX: disable sending IPv6 RA messages from VMs

FIX: disable sending IPv6 RA messages from VMs

There were occasional Router Advertisement packets received in a Traffic
Generator in VM testcases. Fix by adding config that disables sending
IPv6 RA messages on interfaces in VM VPP config before enabling them.

Also rename related keywords and change descriptions based on
https://docs.fd.io/vpp/21.01/db/d4c/clicmd_src_vnet_ip6-nd.html#clicmd_ip6_nd

Change-Id: I1c2a33337ac91f6039d287b4c2aac388e3a52383
Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>