Add a "pass-through-to-hosted-account" option during setup.

[todd]

In the initial setup dialog, the CMS owner is asked to enter a username/password to create User/Admin? asset. However, the ldap-passthu mechanism means that there is not absolute need for such an asset, since the CMS can auth against the TCMS_AUTHFILTER, if configured by the host. This leads to a couple of tasks:

• On the setup screen, if TCMS_AUTHFILTER is set, then show a checkbox with a caption such as 'use my web host account information" and don't create the admin asset in that case.
• for the future ACL system, when a non-asset-based user logs in, create a "passthru" asset for that user (with password set to pass to TCMS_AUTHFILTER).