Do a better job of identifying CentOS 6

Since DIB creates a chroot but doesn't actually chroot in an
extra-data script, look for /etc/redhat-release relative to the
temporary chroot base path. Also given that the centos element
leaves DIB_RELEASE exported as am empty string in the calling
environment rather than unset, place it first in the conditional
list.

Change-Id: I0a51f88ff5ea71b6aa6e6406b656cefb0e135e88

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

@@ -20,20 +20,25 @@ import os
 import subprocess
 import sys
 
+TMP_MOUNT_PATH = os.environ['TMP_MOUNT_PATH']
+TMP_HOOKS_PATH = os.environ['TMP_HOOKS_PATH']
+
 # this is a bit weird; we want to get the filter to use on the
 # devstack lib/[apt|rpm] files to pre-install them.  So we have to
 # swizzle things around
 RELEASE=None
+if 'DIB_RELEASE' in os.environ:
+    RELEASE=os.environ['DIB_RELEASE']
 try:
-    if open('/etc/redhat-release').read().startswith('CentOS release 6'):
+    if open(os.path.join(
+            TMP_MOUNT_PATH,
+            '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:
     if os.environ['DISTRO_NAME'] == 'centos7':
         # centos7 matches as rhel7 in devstack
@@ -42,9 +47,6 @@ if not RELEASE:
     print "Can not determine RELEASE"
     sys.exit(1)
 
-TMP_MOUNT_PATH = os.environ['TMP_MOUNT_PATH']
-TMP_HOOKS_PATH = os.environ['TMP_HOOKS_PATH']
-
 DEVSTACK = os.path.join(TMP_MOUNT_PATH, 'opt/git/openstack-dev/devstack')
 CACHEDIR = os.path.join(TMP_MOUNT_PATH, 'tmp')
 IMAGES=os.path.join(TMP_HOOKS_PATH, 'source-repository-images')