trivial: Update '_get_foo_traits' docstrings
trivial: Update '_get_foo_traits' docstrings

These stated they could return None but that wasn't true. Correct this.

Change-Id: I3bbeedef70e4b94225e1a042cf4e59fb12bcaa92
Signed-off-by: Stephen Finucane <492121341a95b3c3aab646bed44634f739dd019b@redhat.com>

mahak/nova,klmitch/nova,klmitch/nova,openstack/nova,mahak/nova,mahak/nova,klmitch/nova,openstack/nova,klmitch/nova,openstack/nova,rahulunair/nova,rahulunair/nova,rahulunair/nova