Fix push lookups

@mitchellh this is a partial revert of 84ae22e. It took me a little bit
to figure out why this broke everything, but then I finally realized it.

84ae22e changes the finalize! function to lookup pushes by strategy
type, but pushes are keyed by push strategy name. In other words, given:

    config.push.define("foo", strategy: "bar")

the `push_configs` has will look like:


    { :foo => [:bar, #<PushConfig>] }

This is important, because if we key by strategy, the user cannot 
specify the same push strategy more than once:

    config.push.define("foo", strategy: "bar")
    config.push.define("zip", strategy: "bar")

If we keyed off of the strategy, this would be impossible.

This also fixes a couple other errors that were causing the build to fail.
There is already a test helper for this `with_env` in this same file. You should use that instead. It probably isn't as thread-safe, but its safer in that its actually modfying the `ENV`.