Enhance IPv6 scenario - Ping default GW

1. Use common _check_remote_connectivity method to verify connectivity between
servers
Old code was a single try expecting 0% packet loss. Common method is resistant
to packet loss and has retries.
Failure message now includes source and dest. Phrasing matches other tests' to
enhance debug searches.

2. Verify connectivity against default v6 gateway.

Change-Id: I32414ccc9818cb58db1cf2477619b40b7b283a3a
1 file changed