Pin ansible in test-requirements

so that it matches what is tested otherwise.

Change-Id: I4eef9f89f1cd262ca2a96c20079756f5f1499ad3

test-requirements.txt

@@ -13,4 +13,4 @@ coverage!=4.4,>=4.0 # Apache-2.0
 docker>=2.4.2 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
-ansible>=2.9.0 # GPLv3
+ansible>=2.9.0,<2.10 # GPLv3