Pushes are hashed by name, not strategy.

@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.

plugins/kernel_v2/config/push.rb

@@ -21,18 +21,18 @@ module VagrantPlugins
 
         # Compile all the provider configurations
         @__defined_pushes.each do |name, tuples|
-          # Capture the strategy so we can use it later. This will be used in
-          # the block iteration for merging/overwriting
-          strategy = name
-          strategy = tuples[0][0] if tuples[0]
-
           # Find the configuration class for this push
-          config_class = Vagrant.plugin("2").manager.push_configs[strategy]
+          config_class = Vagrant.plugin("2").manager.push_configs[name]
           config_class ||= Vagrant::Config::V2::DummyConfig
 
           # Load it up
           config = config_class.new
 
+          # Capture the strategy so we can use it later. This will be used in
+          # the block iteration for merging/overwriting
+          strategy = name
+          strategy = tuples[0][0] if tuples[0]
+
           begin
             tuples.each do |s, b|
               # Update the strategy if it has changed, reseting the current