Introduce more configuration consistency checks

This patch enhances the duplicate DRBD minors checks (currently just a
few) and adds automatic checks of configuration consistency at
configuration file writing time.

In order to do so and show meaningful error messages, the
_UnlockedComputeDRBDMap function is changed to not raise errors in case
of duplicates, but instead return both the minors map and the duplicate
list, and its callers now raise the error. This allows the VerifyConfig
function to return a complete list of duplicates.

The new checks required some small updates to the unittests for the
config module.

Reviewed-by: ultrotter