add: possibility to have other backends in FakeNagiosEnvironment

This permits to drive FakeNagiosEnvironment with customised start/stop commands.
Instead of adding backend parameter, why not create a new method: create_minimal_shinken_configs() and put all shinken specific additions there ?

Adding a third or 4th backend will always require changes to this file anyway (since config_additions variable lives here) and it will IMHO be cleaner if shinken specific logic lives in a seperate method and this method is dedicated to what it is (the bare minimals)