Forwarding -b and -B options to `create_instance.sh` so that we can run the...

Forwarding -b and -B options to `create_instance.sh` so that we can run the buildtree test directly like this:
```
./run_systemtest.sh  -n toto -s tests/archive_retrieve.sh -O -D -k -b /buildtree -B CTA-build
```
This will be very helpful to test buildtree in CI automatically.