Fix behavior of urpmi module where it always return "changed"
even when a package is already installed, since urpmi will
always return 0 wether it install or not.
optionally use rpm python module instead of calling a
external executable, to avoid the cost of forking.
Since python-rpm is not automatically present, we still fallback
on the slower rpm fork method.
Merge pull request #4931 from mscherer/urpmi_fix

Various fix for the urpmi module