Fix bug where accelerate keyword was not checked for boolean value

The play was just checking for the presence of the keyword in the
YAML datastructure, and not the value of the field, so doing something
like variable substitution was always causing the play to be accelerated

lib/ansible/playbook/play.py

@@ -112,7 +112,7 @@ class Play(object):
         self.gather_facts     = ds.get('gather_facts', None)
         self.remote_port      = self.remote_port
         self.any_errors_fatal = ds.get('any_errors_fatal', False)
-        self.accelerate       = ds.get('accelerate', False)
+        self.accelerate       = utils.boolean(ds.get('accelerate', 'false'))
         self.accelerate_port  = ds.get('accelerate_port', None)
         self.max_fail_pct     = int(ds.get('max_fail_percentage', 100))