Allow Django unknown language in LANGUAGES settings

Language preference form expects all languages in LANGUAGES in
openstack_dashboard/settings.py. This leads to an error when
adding a language not listed in django/conf/locale to LANGUAGES list.

This commit allows such language to LANGUAGES in the settings file.

Closes-Bug: #1229026

Change-Id: I37d7f4b089b0299942f25084f741a557467b3be7
Merge "Allow Django unknown language in LANGUAGES settings"