Update docs for i18n exception
Update docs for i18n exception

Adding commonly used exception to import translation functions directly
from i18n modules.

Added example as well to the import template.

Removed the old reference to oslo-incubator which used to host these
functions.

Change-Id: I8f8e56198e6a5072ef6b98e7315276aad83f3ba3

openstack-dev/hacking,openstack-dev/hacking