refactor manifest.json file into a module constant

[weakcamel]

This is to allow overriding the value (if importing the artifactory_cleanup package from code) to support non-standard manifest file names, e.g. "list.manifest.json" as emitted from 'docker build --cache-to/--cache-from command'

Change inspired by my colleague who worked out how to clean up image caches in the first place.

[allburov]

@weakcamel hi! One small change and we're good to go!