Enable user to specify XEN_INTEGRATION_BRIDGE

This could help to run neutron in the official CI, as we don't pass
around variables through kernel parameters there. With this change, if
the user has a XEN_INTEGRATION_BRIDGE parameter in localrc, that's value
will be used.

Change-Id: Id90c801f010584e81d3b031361468817cda67053
Hack interface names to match CI's setup

This is a proof-of-concept dirty hack. As there are no localrc values to
specify PUBLIC_INTERFACE_DEFAULT and GUEST_INTERFACE_DEFAULT, I
hardcoded the public CI's values here.

Change-Id: I2f2285a502f495709db31e153789df37aec44137