Lock oslo.config on version 1.15.0,

since 2.x, oslo.config have removed compatibility of the old name
oslo.config

requirements.txt

@@ -3,7 +3,7 @@ iso8601
 eventlet
 netaddr>=0.7.6
 pyserial
-oslo.config
+oslo.config==1.15.0
 six>=1.7.0
 Babel>=1.3
 oauthlib