Support multiple configs and make template params self documenting.

Template configs are self documenting and can be seen with `hod_pbs.py --help-templates`

Users can also specify multiple configs. e.g. `hod_pbs.py --config dir1/hod.conf,dir2/hod.conf`. This will merge the configs with the following rules:

1: Strings from the second config override the first one.

2. Lists (e.g. `service=taskrunner.conf,resourcemanager.conf`) will get concatenated so users can specify extra services, modules, env vars and the like.

3. Dicts are merged using a deep merging algorithm.
yes, but it doesn't actually prevent you from running it in an environment that isn't 2.7. so it is your problem (esp if the fix is this simple).