refactoring: make BusType a local type

refactoring: make BusType a local type

BusType does not need to be a global type, because in fact
it is needed only to choose a correct policy checker.

In the test cases, or in the command line tools we explicitly know
which checker we need to choose.

Thus, it is only required for the API, where we still keep it
in the configuration.

This will allow further simplifications.

Change-Id: I61663d496279b47a6a0338b93434b5ae42326f33