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
diff --git a/HACKING.rst b/HACKING.rst
index a546f8c..eafa81b 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -153,6 +153,19 @@
                              kwarg2=dict_of_numbers)
 
 
+Test Skips
+----------
+If a test is broken because of a bug it is appropriate to skip the test until
+bug has been fixed. However, the skip message should be formatted so that
+Tempest's skip tracking tool can watch the bug status. The skip message should
+contain the string 'Bug' immediately followed by a space. Then the bug number
+should be included in the message '#' in front of the number.
+
+Example::
+
+  @testtools.skip("Skipped until the Bug #980688 is resolved")
+
+
 openstack-common
 ----------------