Fix Docker machines being lost in go side

By pruning machines that are in "unknown" status after each operation,
the Go code path exposed the fact that the Docker provider was not
updating the machine index during an "up" - leaving the state as
"unknown".

This is basically a bug within the Docker provider, so I think it's okay
to update the plugin code to correct this rather than working around the
issue in Go.

All we need to do is call `machine.state` instead of reaching through to
`machine.provider.state` while waiting for the container to be started.
That causes the extra logic for updating the machine index in
`machine.state` to fire.

plugins/providers/docker/action/wait_for_running.rb

@@ -16,8 +16,10 @@ module VagrantPlugins
 
           wait = true
           if !machine.provider_config.remains_running
+            @logger.debug("remains_running is false")
             wait = false
-          elsif machine.provider.state.id == :running
+          elsif machine.state.id == :running
+            @logger.debug("container is already running")
             wait = false
           end
 
@@ -28,7 +30,7 @@ module VagrantPlugins
 
           # First, make sure it leaves the stopped state if its supposed to.
           after = sleeper(5)
-          while machine.provider.state.id == :stopped
+          while machine.state.id == :stopped
             if after[:done]
               raise Errors::StateStopped
             end
@@ -38,7 +40,7 @@ module VagrantPlugins
           # Then, wait for it to become running
           after = sleeper(30)
           while true
-            state = machine.provider.state
+            state = machine.state
             break if state.id == :running
             @logger.info("Waiting for container to run. State: #{state.id}")