KAFKA-4251: fix test driver not launching in Vagrant 1.8.6

custom ip resolver in test driver makes incorrect assumption when calling vm.communicate.execute, causing driver to fail launching with Vagrant 1.8.6, due to https://github.com/mitchellh/vagrant/pull/7676

Author: Xavier Léauté <xavier@confluent.io>

Reviewers: Ewen Cheslack-Postava <ewen@confluent.io>

Closes #1962 from xvrl/fix-vagrant-resolver

Vagrantfile

@@ -99,9 +99,11 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
       if !cached_addresses.has_key?(vm.name)
         state_id = vm.state.id
         if state_id != :not_created && state_id != :stopped && vm.communicate.ready?
-          vm.communicate.execute("/sbin/ifconfig eth0 | grep 'inet addr' | tail -n 1 | egrep -o '[0-9\.]+' | head -n 1 2>&1") do |type, contents|
-            cached_addresses[vm.name] = contents.split("\n").first[/(\d+\.\d+\.\d+\.\d+)/, 1]
+          contents = ''
+          vm.communicate.execute("/sbin/ifconfig eth0 | grep 'inet addr' | tail -n 1 | egrep -o '[0-9\.]+' | head -n 1 2>&1") do |type, data|
+            contents << data
           end
+          cached_addresses[vm.name] = contents.split("\n").first[/(\d+\.\d+\.\d+\.\d+)/, 1]
         else
           cached_addresses[vm.name] = nil
         end