Allow overriding of hardcoded siteroot
this will help testing the LbEnv RPM by running something like:
rpm -ivh /tmp/LbEnv-1.1.0-2.gcd8aa2d.noarch.rpm
export OVERRIDE_MYSITEROOT=/cvmfs/lhcb.cern.ch/lib
export OVERRIDE_LBENVROOT=/cvmfs/lhcb.cern.ch/lib/var/lib/LbEnv/unstable/x86_64-centos7
. /opt/LHCbSoft/etc/cern_profile.sh