Enable DIB service to build with minimal net access

Adds two new options to lib/dib that will help devstack slaves
use cached content and content local to cloud provides:

 * DIB_BUILD_OFFLINE: This will enable DIB to rely entirely on
   cached images without making any additional calls to cloud-images.ubuntu.com
   to validate hashes/freshness.

 * DIB_APT_SOURCES: Used to specify alternatve sources.list for image builds.
   Setting this enables the addition of the apt-sources element during image
   builds and is only supported for ubuntu/debian builds.

Change-Id: I9b9ca72ec551565d454610aacb86c585f0384f13
Partial-bug: #1375488

lib/dib

@@ -20,13 +20,28 @@ set +o xtrace
 # set up default directories
 DIB_DIR=$DEST/diskimage-builder
 TIE_DIR=$DEST/tripleo-image-elements
+
+# NOTE: Setting DIB_APT_SOURCES assumes you will be building
+# Debian/Ubuntu based images. Leave unset for other flavors.
+DIB_APT_SOURCES=${DIB_APT_SOURCES:-""}
+DIB_BUILD_OFFLINE=$(trueorfalse False $DIB_BUILD_OFFLINE)
 DIB_IMAGE_CACHE=$DATA_DIR/diskimage-builder/image-create
 DIB_PIP_REPO=$DATA_DIR/diskimage-builder/pip-repo
 DIB_PIP_REPO_PORT=${DIB_PIP_REPO_PORT:-8899}
+
 OCC_DIR=$DEST/os-collect-config
 ORC_DIR=$DEST/os-refresh-config
 OAC_DIR=$DEST/os-apply-config
 
+# Include the apt-sources element in builds if we have an
+# alternative sources.list specified.
+if [ -n "$DIB_APT_SOURCES" ]; then
+    if [ ! -e "$DIB_APT_SOURCES" ]; then
+        die $LINENO "DIB_APT_SOURCES set but not found at $DIB_APT_SOURCES"
+    fi
+    EXTRA_ELEMENTS="apt-sources"
+fi
+
 # Functions
 # ---------
 
@@ -106,9 +121,11 @@ function disk_image_create_upload {
 
     # The disk-image-create command to run
     ELEMENTS_PATH=$elements_path \
+    DIB_APT_SOURCES=$DIB_APT_SOURCES \
+    DIB_OFFLINE=$DIB_BUILD_OFFLINE \
     PYPI_MIRROR_URL=$pypi_mirror_url \
     PYPI_MIRROR_URL_1=$pypi_mirror_url_1 \
-    disk-image-create -a amd64 $image_elements \
+    disk-image-create -a amd64 $image_elements $EXTRA_ELEMENTS \
         --image-cache $DIB_IMAGE_CACHE \
         -o $image_path