Refactor config manager into module singleton

The config manager really deserves to be a singleton, but since Python
does not have access modifiers for class attributes, singletons cannot
be reliably implemented with classes.

However, a module is by nature a singleton, so refactor the config
manager class into a plain module instead.
Extract checksum formatting functions into module

Make use of the new singleton config manager and extract the common
checksum/output formatting functions of the check and add subcommands
into a common helper module.
Merge branch 'feature/make-cfgmgr-into-module-singleton' into develop