Add IPv6 address to bastion hosts

Autoconfigured IPv6 address for bastion hosts are already used in a
number of place like network.pp. Using autoconfigured IPv6 addresses is
not the clearest way of doing things so adding static IP to hosts using
interface::add_ip6_mapped

Change-Id: Iead9659c73cafa2d2a42a88c82a26ec976ecad63