config.py: Use half of cores # for ASAN builds

Address Sanitizer (ASAN) builds are 2x slower and consumes 2x the memory.
Usual build machines configuration is 1-core/2G RAM. For ASAN we need
to run on half of cores or 1-core/4G RAM. Otherwise we might kill the
machine.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #6 from davidlt/config-asan-builds

config.py: Use half of cores # for ASAN builds