Use StrOpt's parameter choices to restritct option auth_strategy

oslo.config provides value check for StrOpt with parameter choices.
And we use option auth_strategy directly without any check like:
 limit_name = CONF.auth_strategy + '_nolimit'
So we'd better restrict its value in keystone and noauth2.

Change-Id: I782c493d43fcdd27c3d6660cbb2800c90fbbd980

nova/api/auth.py

@@ -36,6 +36,7 @@ auth_opts = [
                      'is removed from v2.1 api.'),
     cfg.StrOpt('auth_strategy',
                default='keystone',
+               choices=('keystone', 'noauth2'),
                help='''
 The strategy to use for auth: keystone or noauth2. noauth2 is designed for
 testing only, as it does no actual credential checking. noauth2 provides