pylint: set default jobs limit to use all available cpus

Since pylint-1.4 (released 2014), pylint has offered a --jobs option
to process multiple files in parallel.  Change our pylintrc default
to autodetect all available cpus.

Bug: None
Test: running `./tools/pylint.py *.py` is faster
Change-Id: Id15573bc2e8b0cfdaaa3623fc716734687d3f26c