correctly disable parallel build for ATLAS

the `set_parallellism` method is moved and renamed as a part of https://github.com/hpcugent/easybuild-framework/pull/953, since it's not considered to be part of the easyblocks API

This patch tweaks the ATLAS easyblock to properly disable a parallel build.
This message makes no sense :wink:
Just drop it? if the parallel is already on 1, there is no issue. Only display the warning when it is needed?