Renumber some nova hacking checks

Most of the nova hacking checks had picked numbers starting
from approx N300 onwards. Two recent additions randomly
picked N123 and N500. Renumber these to N313 and N314 and
make sure they're documented in HACKING.rst. Mention the
guidelines in the hacking source file for benefit of
future authors

Change-Id: Ia3eb4cb9a4ac7409db7eba9e1689f4a5780b8795
Merge "Renumber some nova hacking checks"