Switch to new concretizer config
In recent installs using Spack 0.18.1 a warning message is issued for many commands:
==> Warning: concretization:together is deprecated and will be removed in Spack 0.19 in favor of the new concretizer:unify:true config option.
Unless there are pre-0.18 installs around (CINECA?), we should switch to the new concretizer.