Wrong usage of "a"

Wrong usage of "a" in the mesages:
"a integer representing the number of matched stacks"
"Send a signal and a exception will raise"
Should be:
"an integer representing the number of matched stacks"
"Send a signal and an exception will raise"
Totally 3 occasion in Heat base code.

Change-Id: Ic5ff16dd8d31d4573490ac423f7457e75b872e6b
diff --git a/functional/test_autoscaling.py b/functional/test_autoscaling.py
index df8c54b..d3448e3 100644
--- a/functional/test_autoscaling.py
+++ b/functional/test_autoscaling.py
@@ -729,7 +729,7 @@
         self._wait_for_resource_status(
             stack_identifier, 'JobServerGroup', 'SUSPEND_COMPLETE')
 
-        # Send a signal and a exception will raise
+        # Send a signal and an exception will raise
         ex = self.assertRaises(exc.BadRequest,
                                self.client.resources.signal,
                                stack_identifier, 'ScaleUpPolicy')