Add hacking checks

Checking specific mistakes can be automated via hacking rules.

This patch:
 * adds HACKING.rst - list of rules;
 * adds rally.hacking - module for implementation of rules;
 * edits tox.ini, so hacking rules will be checked during pep8 test;
 * adds 3 rules related to mistakes in  `assert_*` methods from `mock`
   library. Also tests, which are failed due to these rules, are fixed.

Closes-Bug: #1305991
Change-Id: I605dd5cfba4eb83d0735e4f9f3ab4e44e149d041
Merge "Add hacking checks"