Add option Default Group for Users in LDAP authentication UI.

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
Add test cases for User. authenticate_ldap.

https://bugzilla.redhat.com/show_bug.cgi?id=1060207
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
Fixed show/hide of ldap_default_group_div & updated pull-down choices.

- Fixed show/hide of ldap_default_group_div
- Changed 'Default Group for Users' pull-down choice <Choose> to <No Default Group>

https://bugzilla.redhat.com/show_bug.cgi?id=1060207
Merge pull request #778 from lfu/default_group_for_ldap_users_1060207

Support Default Group for Users with LDAP authentication.