New staging for GCC7
A new gcc version shall be prepared / tested / integrated.
In the same way we handled GCC6, we should prepare openSUSE:Factory:Staging:Gcc7 (basically a normal staging similar to A-C, doing a full ring 0 - 2 rebuild, but due to it's long-living nature, out of the 'regular' staging namespace to no block us.
- setup Gcc7 staging
- adjust create_test_dvds.sh for Gcc7