cvmfs: specify which client version to be installed

[CatalinCondurache]

The current template
template-library-standard/features/cvmfs/rpms.pan
doesn't allow a staged rollout of a new client release across of a batch farm. Once the latest package is added to the repository, any 'aq make' on a worker node for a separate and different fix will install the cvmfs-client package as well.

I propose to replace
'/software/packages/{cvmfs}' ?= dict();
with something else i.e. pkg_repl where one can make use of CVMFS_CLIENT_VERSION

Thanks,
Catalin