Determine CentOS 6 platform in cache-devstack

The centos element (which is CentOS 6.x specific) doesn't provide
DIB_RELEASE so we have to identify it another way.

Change-Id: Ibf3b25b99f03c6077538dec6c3770dc0f690e3f5

nodepool/elements/README.rst

@@ -28,7 +28,7 @@ Building an image is simple, we have a script!
 
 ::
 
-  DISTRO="ubuntu" DIB_RELEASE="trusty" bash tools/build-image.sh
+  DISTRO="ubuntu" bash tools/build-image.sh
 
 See the script for environment variables to set distribution, etc.
 You should be left with a .qcow2 image file of your selected

nodepool/elements/cache-devstack/extra-data.d/55-cache-devstack-repos

@@ -24,6 +24,14 @@ import sys
 # devstack lib/[apt|rpm] files to pre-install them.  So we have to
 # swizzle things around
 RELEASE=None
+try:
+    if open('/etc/redhat-release').read().startswith('CentOS release 6'):
+        # not actually used in devstack, but for consistency and because this
+        # script will error if run on a platform where RELEASE isn't detected
+        # as something
+        RELEASE='rhel6'
+except IOError:
+    pass
 if 'DIB_RELEASE' in os.environ:
     RELEASE=os.environ['DIB_RELEASE']
 if 'DISTRO_NAME' in os.environ: