Store global pool manager instance in settings

After 1.6, we can no longer rely on the Vagrant Environment instance to persist
plugin-specific state. This means that values set during environment
initialization hooks will not survive to be present for the actions that need
them.

This patch moves the storage of state to the AutoNetwork module its self.

(Also, integration tests. Yey!)

Ref: mitchellh/vagrant#3660
Update LoadPool Action for Vagrant 1.6

As of 1.6, actions called from the `:environment_load` hook no longer have
access to some information. The workaround is to use the `env[:env]` key.

The situation is complicated by the fact that actions called outside of
`:environment_load` may be passed an environment that does not inclue
`env[:env]`. The fix for these situations is to only run the `LoadPool` action
during the `:environment_load` hook.

Ref: mitchellh/vagrant#3701
Merge pull request #20 from Sharpie/fix-1_6-stuff

Fixes for Vagrant 1.6

Fixes GH-17.