Add `singularity.autoMounts = true` to config or put `autoMounts = true` in scope

Hi,

after my `sarek` runs aborted due to what looked like an issue with `singularity` autoMounts, I figured this line in `nextflow.config` 

https://github.com/nf-core/sarek/blob/508735eb09d2fe33e904516db8db128b0a808dd2/nextflow.config#L118 

should either be `singularity.autoMounts = true` or put in a scope.

Hope this helps.
Federico
