Tweak to the way serial percentages are handled

lib/ansible/executor/playbook_executor.py

@@ -227,7 +227,10 @@ class PlaybookExecutor:
             serial_pct = int(play.serial.replace("%",""))
             serial = int((serial_pct/100.0) * len(all_hosts))
         else:
-            serial = int(play.serial)
+            if play.serial is None: 
+                serial = -1
+            else:
+                serial = int(play.serial)
 
         # if the serial count was not specified or is invalid, default to
         # a list of all hosts, otherwise split the list of hosts into chunks

lib/ansible/playbook/play.py

@@ -87,7 +87,7 @@ class Play(Base, Taggable, Become):
     _any_errors_fatal    = FieldAttribute(isa='bool', default=False, always_post_validate=True)
     _force_handlers      = FieldAttribute(isa='bool', always_post_validate=True)
     _max_fail_percentage = FieldAttribute(isa='percent', always_post_validate=True)
-    _serial              = FieldAttribute(isa='percent', default=0, always_post_validate=True)
+    _serial              = FieldAttribute(isa='string',  always_post_validate=True)
     _strategy            = FieldAttribute(isa='string', default='linear', always_post_validate=True)
 
     # =================================================================================