Make list_images.sh emit the etcd3 tarball

We need this for every devstack run now, so downloading it from github
every time isn't the most awesome thing in the world.

Add an extra variable EXTRA_CACHE_URLS which will be appended to the
output of tools/image_list.sh.  This way, these files will be
downloaded during the daily nodepool build, but they will not be in
the IMAGE_LIST and hence be considered as images to upload.

Add a function get_extra_file which echos the path to a file given the
URL.  It will first check the cache at $FILES, and if not present
download it.

Update the documentation in image_list.sh to reflect what's happening.

Move the defaults for etcd variables into stackrc, since it is a base
service now.

Change-Id: I86104824a29d973a6288df1f24b7891feb86267c

functions

@@ -45,6 +45,37 @@ function short_source {
 # export it so child shells have access to the 'short_source' function also.
 export -f short_source
 
+# Download a file from a URL
+#
+# Will check cache (in $FILES) or download given URL.
+#
+# Argument is the URL to the remote file
+#
+# Will echo the local path to the file as the output.  Will die on
+# failure to download.
+#
+# Files can be pre-cached for CI environments, see EXTRA_CACHE_URLS
+# and tools/image_list.sh
+function get_extra_file {
+    local file_url=$1
+
+    file_name=$(basename "$file_url")
+    if [[ $file_url != file* ]]; then
+        # If the file isn't cache, download it
+        if [[ ! -f $FILES/$file_name ]]; then
+            wget --progress=dot:giga -c $file_url -O $FILES/$file_name
+            if [[ $? -ne 0 ]]; then
+                die "$file_url could not be downloaded"
+            fi
+        fi
+        echo "$FILES/$file_name"
+        return
+    else
+        # just strip the file:// bit and that's the path to the file
+        echo $file_url | sed 's/$file:\/\///g'
+    fi
+}
+
 
 # Retrieve an image from a URL and upload into Glance.
 # Uses the following variables: