prefer-ipv6 passed through to agent config
Merge pull request #307 from dimitern/prefer-ipv6-agent-config

Pass prefer-ipv6 flag through agent config

This builds upon the previous PR adding the prefer-ipv6 environ
config setting. Now, the prefer-ipv6 flag is passed through further
to the agent config, so the agents can initialize the network package
with it even before connecting to the API/state servers (no environ
config yet).
Merge branch 'master' of https://github.com/juju/juju into doc-design-api-spec