IPv6: refactor Collect VM IPv6 SLAAC Addresses
Make Collect VM IPv6 SLAAC Addresses test more reliable: at first it
will take IPv6 address, which was saved for given VM in neutron
database, after that it will try to connect to VM at this address and
check an output of ip -6 a command.
Update arguments for 'Collect VM IPv6 SLAAC Addresses' call in
netvirt/vpnservice/vpn_basic_ipv6.robot test suite.
Change-Id: I4a1df2bd97372e60f361ffd5442b945a1cc528db
Signed-off-by: Valentina Krasnobaeva <valentina.krasnobaeva@6wind.com>