Varnish cache_file name is optional

[donaldgray]

Main change was to introduce VARNISH_CACHE_FILE envvar, which is optional. Previously VARNISH_CACHE_FOLDER was used as location and varnish_cache.bin as name for file. However, by making VARNISH_CACHE_FILE optional we can just specify a folder and Varnish will create a file of it's own.

From varnish docs:

The ‘path’ parameter specifies either the path to the backing file or the path to a directory in which varnishd will create the backing file.

Other changes:

• Update base image to more up to date Ubuntu version
• Change order of some statements in dockerfile to prevent need to install pypi requirements if start.sh updated
• Update start.sh to allow it to run without cleanup_handler if no INCOMING_QUEUE specified.
• Update boto requirements as docker image failing to build.
• Moved requirements that are only for testing to a separate test_requirements.txt to avoid installing on docker image.