Add os-collect-config to requirements

The test that uses os-collect-config, ZaqarSignalTransportTest, is
staying in heat-tempest-plugin and being removed from
heat_integrationtests in the Heat tree. Add it to requirements here so
we can drop it from test-requirements there.

Change-Id: Ieadd9ecac0df4e921f96282f4f807df7ff81c26f
diff --git a/requirements.txt b/requirements.txt
index f030897..34fa1fd 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,6 +6,7 @@
 oslo.config>=5.1.0 # Apache-2.0
 oslo.log>=3.36.0 # Apache-2.0
 oslo.messaging>=5.29.0 # Apache-2.0
+os-collect-config>=5.0.0 # Apache-2.0
 paramiko>=2.0.0 # LGPLv2.1+
 python-cinderclient>=3.3.0 # Apache-2.0
 gnocchiclient>=3.3.1 # Apache-2.0