CSIT: add dualstack test for one-router solution

CSIT: add dualstack test for one-router solution

Introduce new basic dualstack vpnservice test-suite to check
support of dualstack VMs inside L3 BGPVPN. Implemented
neutron network config includes one dualstack router.
It allows to test one-router solution, defined in
l3vpn-dual-stack-vm.rst specification: commit
07ef3e3db56610a878e0a00ada175fa70f05ffe6 "Spec to support
dualstack VMs in L3VPN"

Change-Id: Ie9b048c1ce7329680a064d6fca572b6930244d71
Signed-off-by: Valentina Krasnobaeva <valentina.krasnobaeva@6wind.com>
Signed-off-by: Sam Hague <shague@redhat.com>
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>