Fix for serial when percent amount is less than one host (#15396)

Ansible when there was a percentage that was calculated to be less than
1.0 would run all hosts as the value for a rolling update.

The error is due to the fact that Python will round a
float that is under 1.0 to 0, which will trigger the case of
0 hosts. The 0 host case tells ansible to run all hosts.

The fix will see if the percentage calculation after int
conversion is 0 and will else to 1 host.

lib/ansible/executor/playbook_executor.py

@@ -214,7 +214,7 @@ class PlaybookExecutor:
         # and convert it to an integer value based on the number of hosts
         if isinstance(play.serial, string_types) and play.serial.endswith('%'):
             serial_pct = int(play.serial.replace("%",""))
-            serial = int((serial_pct/100.0) * len(all_hosts))
+            serial = int((serial_pct/100.0) * len(all_hosts)) or 1
         else:
             if play.serial is None:
                 serial = -1