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
d8bb220606