Add pylint-based lintstack test to tox environment

lintstack is a pylint-based static analysis tool to find bugs
in Python code.

To run the test via tox: $ tox -e pylint
To run the test directly: $ tools/lintstack.sh

Change-Id: If155f47ed84eb820ac505ee741bdd18edd41b1eb
Merge "Add pylint-based lintstack test to tox environment"