admin: Fix auth provider filters

Due to a bug in Django (https://code.djangoproject.com/ticket/14645), it
is not possible to use the "HAS NOT" filter for auth providers using the
`exclude` method. Fix this behavior using a subquery to filter the users
that have the requested auth provider and then exclude them.

Fix apyrgio/synnefo#199
admin: Fix default value for negative filters

Filters such as the filter for Auth providers exclude results and should
not have as default value "All". Change it to "None" for this cases.
Merge branch 'feature-admin-fix-filters' into feature-admin