Add support for using cvmfs when it is not installed on a node
Enhance GlideinWMS to use https://github.com/cvmfs-contrib/cvmfsexec to mount cvmfs repositories when cvmfs is not installed on a worker node. It requires unprivileged user namespaces (optional on el7, default on el8) and the ability for unprivileged users to mount fuse filesystems (using fusermount from the fuse package on el7, default on el8). On el7, the target batch system must not immediately use kill -9 when shutting down a job; it must give some time for a catchable signal to clean up the fuse mountpoints. On el8 the mountpoints will be cleaned up even with kill -9.