Add configure-cache tool to stackinator

[Ben Cumming]

Add a command line tool for configuring caches in recipes, based on the tool used in the alps spack stacks:

usage: configure-cache [-h] [-k KEY] [-p PATH] [-d] [--read-only] recipe

Configure a stackinator recipe to enable/disable a build cache.

positional arguments:
  recipe                the path of the recipe to configure

optional arguments:
  -h, --help            show this help message and exit
  -k KEY, --key KEY     path to the gpg key used to sign packages - required
                        to update the cache.
  -p PATH, --path PATH  path to the build cache - if provided without a key
                        the cache will be read only.
  -d, --disable         disable the build cache
  --read-only

To consider:

• extending the stack-config command line to accept cache configuration options
• supporting a JSON file format for specifying cache configurations
• support for creating pushing to a sub-directory in a cache for each mount point