Initial centos7 support for build-image.sh

Add some filtering to 55-cache-devsatck-repos to handle centos7, which
actually gets setup with a DISTRO_NAME of rhel7 in devstack

Update the readme file to highlight the options for building other
images.

"DISTRO=centos7 ./tools/build-image.sh" creates a workable centos7
qcow image with this change and a d-i-b that includes
Ie24033468b78587ea87188ee1b843b26895798ff

Change-Id: Ida38cba0a303ab77432d5d354f1952e00ebfa00e

nodepool/elements/README.rst

@@ -12,7 +12,7 @@ Install the dependencies:
 
 ::
 
-  sudo apt-get install kpartx qemu-utils curl
+  sudo apt-get install kpartx qemu-utils curl python-yaml
 
 Install diskimage-builder:
 
@@ -28,9 +28,15 @@ Building an image is simple, we have a script!
 
 ::
 
-  bash tools/build-image.sh
+  DISTRO="ubuntu" bash tools/build-image.sh
 
-You should be left with a file called devstack-gate-precise.qcow2.
+See the script for environment variables to set distribution, etc.
+You should be left with a .qcow2 image file of your selected
+distribution.
+
+It is a good idea to set ``TMP_DIR`` to somewhere with plenty of space
+to avoid the disappointment of a full-disk mid-way through the script
+run.
 
 Mounting the image
 ------------------

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

@@ -20,8 +20,20 @@ import os
 import subprocess
 import sys
 
+# 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']
+if 'DISTRO_NAME' in os.environ:
+    if os.environ['DISTRO_NAME'] == 'centos7':
+        # centos7 matches as rhel7 in devstack
+        RELEASE='rhel7'
+if not RELEASE:
+    print "Can not determine RELEASE"
+    sys.exit(1)
 
-RELEASE = os.environ['DIB_RELEASE']
 TMP_MOUNT_PATH = os.environ['TMP_MOUNT_PATH']
 TMP_HOOKS_PATH = os.environ['TMP_HOOKS_PATH']