Add oslo.policy requirement to requirements.txt

This requirement should be added to requirements.txt
as it's used by the Patrole framework.

Change-Id: I1fc49e6d3012861760156da851f255c7ac5bad3f
diff --git a/requirements.txt b/requirements.txt
index 6d2c7f1..6871057 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,5 +6,6 @@
 urllib3>=1.15.1 # MIT
 oslo.log>=3.11.0 # Apache-2.0
 oslo.config>=3.22.0  # Apache-2.0
+oslo.policy>=1.17.0  # Apache-2.0
 tempest>=14.0.0  # Apache-2.0
 stevedore>=1.20.0  # Apache-2.0