In ctsm python library: get more machine defaults from config_machines.xml

This issue refers to code that is not yet on master: #493 .

Currently, I have started a machine_defaults.py that gives defaults for some parameters that we need for supported machines. Long-term it would be good to get as much as possible from config_machines.xml, via cime functions. I just haven't taken the time to figure out how to do this right now. This would avoid duplication, reduce maintenance, and allow our python stuff to run on other supported machines without our needing to introduce our own defaults for (e.g.) the scratch directory on each machine.

Specifically: Try to (a) determine machine name based on the existing nodename_regex: is there an easy-to-use cime function for that?; (b) use CIME_OUTPUT_ROOT rather than having our own defaults for the scratch directory on a machine; (c) see if we could also replace other aspects of our machine_defaults.py with stuff from cime (if not, consider whether we'd want to move some of this stuff into config_machines eventually).