Fix KeysView object does not support indexing

The KeysView we get from the hostvars is not indexable and throws an
error [1] when trying to access via an index. This can be fixed by
using an iterator and getting one item.

[1] Log:

  [WARNING]: Failure using method (v2_playbook_on_play_start) in callback plugin
  (<ansible.plugins.callback.zuul_stream.CallbackModule object at
  0x7f5d41b73b70>): 'KeysView' object does not support indexing

Change-Id: I0c95b55a3b26856e9d101db95682efa0e0694523
diff --git a/zuul/ansible/callback/zuul_stream.py b/zuul/ansible/callback/zuul_stream.py
index c4473da..de83248 100644
--- a/zuul/ansible/callback/zuul_stream.py
+++ b/zuul/ansible/callback/zuul_stream.py
@@ -161,7 +161,7 @@
         # Get the hostvars from just one host - the vars we're looking for will
         # be identical on all of them
         hostvars = self._play._variable_manager._hostvars
-        a_host = hostvars.keys()[0]
+        a_host = next(iter(hostvars.keys()))
         self.phase = hostvars[a_host]['zuul_execution_phase']
         if self.phase != 'run':
             self.phase = '{phase}-{index}'.format(