Add missing keystoneauth1 to requirements.txt

The keystoneauth1 library is imported by the Barbican client manager but
was missing in the requirements.txt file.

Change-Id: I75c648026d43d533b21367f3adf77014b07e89a3
diff --git a/requirements.txt b/requirements.txt
index 3b78bc3..8020630 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -17,3 +17,4 @@
 six>=1.10.0 # MIT
 tempest>=17.1.0 # Apache-2.0
 tenacity>=4.4.0 # Apache-2.0
+keystoneauth1>=3.3.0 # Apache-2.0