Make cluster name optional in BOSCO tools and submit files. #3593

Where there is just one cluster registered with BOSCO, the cluster name
no longer needs to be given on the command line of bosco_cluster
commands, and grid_resource doesn't need to be given in the submit file.
The submit file part is achieved using the SUBMIT_EXPRS config param.
bosco_cluster --test now tests all clusters if no name is given.
bosco_cluster --list now takes an optional argument that controls what
it prints for each cluster.
bosco_cluster now has an explicit limit of 254 clusters.