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
1 file changed