Environment variable propagation

[rountree]

When using ssh/rsh to spin up servers, environment variables are propagated from the front end node.

Add, e.g., a -e flag to spindle to allow the user to specify which environment variables to propagate.

The issue first came up when testing slurm+ssh in an ubuntu environment where TMPDIR was not set. Adding TMPDIR to the front end node was not sufficient as it wasn't sent along to the servers.