AuthSettingsController: assume 'auth_plugins' option exists

In formglobals, the provided 'auth_plugins' default value is always
replaced with the real database value, returned along with other
plugin settings by Setting.get_auth_settings.

We can safely assume that the database value exists, since not only does
db_manage initialize the 'auth_plugins' option upon database creation,
but other authentication modules already assume its existence.

For instance, Settings.get_auth_plugins() is equivalent to

Settings.get_auth_settings()['auth_plugins']

except that get_auth_plugins will throw AttributeError, not KeyError,
if the 'auth_plugins' key does not exist in the database.