openstack-repos: fail on missing project list

If you specify a CUSTOM_PROJECTS_LIST_URL that doesn't exist you get a
message but the script continues.  Since you clearly meant to specify
something we should bail so the user can correct the problem.

Additionally, add some documentation to the build-image.sh script to
describe how to use this to cut down build times considerably for
initial testing purposes.

Change-Id: I02d1cd341f793471b1ad4374e98031ea53db7f49

nodepool/elements/openstack-repos/extra-data.d/50-create-repo-list

@@ -36,7 +36,7 @@ def get_project_list(url):
         return projects
     except URLError:
         print "Could not open project list url: '%s'" % url
-        return None
+        raise
 
 def main():
     projects = []

tools/build-image.sh

@@ -14,6 +14,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+#
+# Read through this script.  There are a lot of comments that might
+# help you!
+#
+
 # To run on Ubuntu 14.04, this depends on:
 # diskimage-builder
 # qemu-utils
@@ -59,6 +64,21 @@ fi
 ## break into the grub console.  Or set it to 0 for fast boot.
 # export DIB_GRUB_TIMEOUT=10
 
+## The openstack-repos element caches every git repo, wihch can take
+## quite some time.  We can override this, but some minimal repos are
+## required for a successful build.  For speeding up builds when
+## you're testing the following should work, but be very careful
+## stripping things out when generating real images.
+
+## add to /tmp/custom_projects.yaml:
+##
+## - project: openstack-infra/project-config
+## - project: openstack-infra/system-config
+## - project: openstack-dev/devstack
+
+# export DIB_CUSTOM_PROJECTS_LIST_URL='file:///tmp/custom_projects.yaml'
+
+
 # The list of elements here should match nodepool/nodepool.yaml
 disk-image-create -x --no-tmpfs -o $IMAGE_NAME \
     $DISTRO \