changed 'string' to 'percent' for _serial

Note that this allows both integers (e.g. 3) and percentages (e.g. "30%")
Also changed default back to 0 rather than '0'

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='string', default='0', always_post_validate=True)
+    _serial              = FieldAttribute(isa='percent', default=0, always_post_validate=True)
     _strategy            = FieldAttribute(isa='string', default='linear', always_post_validate=True)
 
     # =================================================================================