Add support for caching the Singularity cached images directory

[natefoo]

Using the cache directory modification time. The container resolvers config documentation is a mess, I am going to remove the sample XML and document it properly in the admin docs in a subsequent PR. For now, you can enable it with a container resolvers config (either in container_resolvers_conf.yml or the container_resolvers dict in the galaxy section of galaxy.yml) with:

- type: cached_mulled_singularity
  cache_directory: /cvmfs/singularity.galaxyproject.org/all
  cache_directory_cacher_type: dir_mtime

I'm going to see what I broke and then once I fix those I'll add a test for the new option.

Fixes #12514

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