Re-establish Backend::createBackend

This partially reverts fcf7f7d6. The original intention was to have
no library code reading config. Moving code snippets to another lib
doesn't solve this. We need quick & easy access to a monitoring
backend, on CLI, in other modules and so forth. Duplicating so much
code again and again is not an option.

I prefer the former way of doing things unless we find a better
solution.
Remove createBackend() method from the base controller

Refering to d389d8e we'll use Backend::createBackend() for accessing monitoring backends.