Improve fallback provision of nodelist.

[shuds13]

The default for auto-resources is to look for environment variables specifying the nodelist. The names of these environment variables can be specified by the user, if they are different to the defaults. However, some systems may provide resource lists through a file (e.g. Cooley).

Currently, if a file called worker_list is present in the run directory, then libensemble will take this as a list of nodes available to the workers. We need to have a better way for the user to provide the complete list of nodes available to libensemble via a file.

This could be done either by having a node_list file in the run directory, or setting an libe env variable to the name of a nodelist file or by providing as a command line option via parse_args. The latter two could have separate options to specify an env_variable

[shuds13]

Solved by #455