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

@Fryguy It does not have to be an array of arrays.
When the elements of the array respond to first and last (such as a two-element array), the “lasts” serve as option values and the “firsts” as option text. Otherwise the elements are used as both option values and option text.