FFe - Active Directory integration

This feature adds support for Active Directory to the GTK UI.

It adds a checkbox on the user setup page to enable active directory. When this checkbox is selected, a second page is displayed on continue to enter the details of the connection to the directory.
A button to test the connection is available on this page.

During installation, the target is configured to join the AD with 'realm' and pam_mkhomedir is setup so on boot, a user of the AD can login directly.

An error during installation is not critical and a message is displayed to explain that the administrator must troubleshoot and perform the configuration of AD on the installed system.

The main user is kept for administrative purpose.
We do not add the AD administrator to the sudo group.

We tested manually positive and negative scenarios on Ubuntu Desktop:
- Standard installation w/o AD selected
- Standard installation with AD and correct connection information
- Standard installation with AD and wrong connection information (wrong domain, wrong user or wrong password)
- Failure of pam_mkhomedir

We also tested on Kubuntu Desktop to ensure that the AD checkbox is not displayed and that installation is still performing as expected.

Dependencies for this features (sssd, realmd, adcli and sssd-tools) are already seeded on Ubuntu Desktop.

If 'realmd' is not installed on the system, the AD checkbox is not displayed and it is not possible to configure a connection.