AC_ARG_VAR with defaults and lsf.cluster.CLUSTERNAME file

While trying to automate the change of cluster name in build/install process I found specifying this at configure time using didn't adequately work. i.e.
```bash
./configure openlavacluster=mycluster --prefix=/software/openlava
make
make install
```
The attached fixes this and adds documentation to the ```./configure --help``` so that the example above is functional, while the default is to use openlava for the clustername and openlava for the openlava admin user - just as currently implemented.