Allow new platform ids when building against LCG nightlies

With the switch to platform ids with microarchitecture levels (LHCbNightlyConf!625 (merged)) we fail to match the platform we want to build with the corresponding LCG nightly build platform. See LHCbNightlyConf#10 (closed)

We have to add a different way to parametrize slot configurations based on the platform id.