Use decimal.Decimal directly

decimal.Decimal was introduced in python 2.4. Since we require at least Python
2.6 for kiwi, there's no need to do workarounds on it anymore.  Also, try to
use 'import decimal' instead of 'from decimal import Decimal' as much as
possible. Only tests are still using the 'from decimal ...' to avoid lots of
changes there.

Change-Id: Ic205d1353ea914c34f9a96af3d9a5f8b15c5500b
Merge "Use decimal.Decimal directly"