Allow possibility to use a variant symlink to select the arch

[ocaisa]

We should a variable symlink so that people can use the cvmfs configuration file to set the arch...that way you would have an invariant path that could be used in scripts even in heterogeneous environments.

@bedroge has actually tried this and has given the example of

I've set up two variable symlinks in the root of our repo
modules -> $ARCH/modules
and
software -> $ARCH/software
That indeed allows you to set $ARCH in the CVMFS config of your nodes.
For instance, ARCH=x86_64/intel/haswell