Update hacking.py for @testtools.skip() formatting.

This commit updates the hacking rules to add a strict format for bug
skips. Previously, there was no defined rules for skip formatting
which caused a number of test skips to be added without consistent
formatting. These skips then failed to get picked up by
tools/skip_tracker.py. This commit adds a new hacking test to ensure
that any skips added conform to a format that the skip_tracker will pick
up. HACKING.rst was also updated to explain the new rules being enforced.

Change-Id: I95f3ec7de2ee5e2039d53ad9565b5cec936a7672
Merge "Update hacking.py for @testtools.skip() formatting."