removed default from hosts to make it requried

prevents writing a play w/o a hosts entry which would default to
all/empty

lib/ansible/playbook/play.py

@@ -64,7 +64,7 @@ class Play(Base, Taggable, Become):
 
     # Connection
     _gather_facts        = FieldAttribute(isa='bool', default=None, always_post_validate=True)
-    _hosts               = FieldAttribute(isa='list', default=[], required=True, listof=string_types, always_post_validate=True)
+    _hosts               = FieldAttribute(isa='list', required=True, listof=string_types, always_post_validate=True)
     _name                = FieldAttribute(isa='string', default='', always_post_validate=True)
 
     # Variable Attributes