Fix replacing host-os for online's plus nodes
On plus nodes LD_LIBRARY_PATH is set to a value which appears to be similar to when compiling for SLC6 from CentOS 7. As a result the SLC substitution is applied erroneously. This commit fixes the issue by requiring the target-os platform to come from a path in /cvmfs/.
See #2 (closed) for details.
Edited by Chris Burr