core: poll for process exit on SSH#exec with subprocess

lib/vagrant/util/ssh.rb

@@ -174,7 +174,13 @@ module Vagrant
         process = ChildProcess.build("ssh", *command_options)
         process.io.inherit!
         process.start
-        process.wait
+
+        # Poll for exited rather than call #wait because #wait will hold
+        # the GIL, locking up the entire Ruby VM. See ChildProcess #68
+        while !process.exited?
+          sleep 0.2
+        end
+
         return process.exit_code
       end
     end