do not erroneously set gathered_facts=True

In `lib/ansible/executor/play_iterator.py`, ansible sets a host's
`_gathered_facts` property to `True` without checking to see if there
are any tasks to be executed.  In the event that the entire play is
skipped, `_gathered_facts` will be `True` even though the `setup`
module was never run.

This patch modifies the logic to only set `_gathered_facts` to `True`
when there are tasks to execute.

Closes ansible#15744.

lib/ansible/executor/play_iterator.py

@@ -295,10 +295,10 @@ def _get_next_task_from_state(self, state, host, peek):
                         setup_block = self._blocks[0]
                         if setup_block.has_tasks() and len(setup_block.block) > 0:
                             task = setup_block.block[0]
-                        if not peek:
-                            # mark the host as having gathered facts, because we're
-                            # returning the setup task to be executed
-                            host.set_gathered_facts(True)
+                            if not peek:
+                                # mark the host as having gathered facts, because we're
+                                # returning the setup task to be executed
+                                host.set_gathered_facts(True)
                 else:
                     # This is the second trip through ITERATING_SETUP, so we clear
                     # the flag and move onto the next block in the list while setting