singletonmixin: Remove singleton mixin

There is no need for a Singleton class in Python,
modules fulfil the same purpose.

* clidisplay: Remove singleton for CliDisplay
* term: Turn TerminalController into a module
* cache: Turn WCache singleton into module
* idmgmt: Turn IdMgmt singleton into module
* schema: Convert schema singleton to module
* msg: ErrorBuffer doesn't need to be a singleton
* report: Report doesn't need to be a singleton
* cibconfig: CibFactory doesn't need to be a singleton
* cibstatus: Remove usage of Singleton
* singletonmixin: Remove the singleton module as it is no longer used
cibconfig: Eliminate external use of is_cib_sane()
dev: Rename vars to constants

The module name vars collides with the built-in function
vars(), which for example prevents linting tools from detecting
when vars is used without having been imported, but also makes
it needlessly difficult to use the standard function while also
using the module.