In case of setting subnet driver to 'namespace', and making use of IPv6
in kuryr devstack‚ we experienced a clash with devstack route, since we
used to use shared subnet pool crated by devstack.
To avoid such clash, for IPv6 we simply create our own IPv6 shared
subnet pool, and subnets within it.
Change-Id: Iad40167d28078b2d6811d3afed58b8da4b41cd42
ec88d2aabf