Support Default Group for Users with LDAP authentication.

When Default Group for Users is enabled in Configure - Configuration - Authentication,
this default group can be used for all ldap users who use LDAP for authentication only,
for example, when an LDAP server does not have memberof attribute for groups.
    
https://bugzilla.redhat.com/show_bug.cgi?id=1060207

This doesn't look it builds correctly.  I thought options_for_select needed an Array of Arrays, but this would look like: 

```ruby
[["<Choose>", nil], "group description 1", "group description 2"]
```