Handle debug tasks more robust in zuul_stream

We currently rely on detecting debug tasks by checking if there is
only a single variable other than variables starting with
'_ansible'. This doesn't work anymore with ansible 2.4. Detecting them
by the action should work better and more reliably.

Further also strip the 'changed' variable in this case as this has no
meaning for a debug task.

Change-Id: I562329ce68c279965afc32ba5fda6615423de43a
1 file changed