gplazma2-htpasswd: Add htpasswd support

This plugin adds support for the htpasswd file - a file format
used by many web servers.

The plugin only supports MD5 hashes. SHA is not supported
at the moment.

The htpasswd can be managed with the htpasswd utility available on
most OSes.

system-test is updated to use htpasswd + authzdb for the admin
account if the htpasswd utility is available.

Target: trunk
Require-book: yes
Require-notes: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: http://rb.dcache.org/r/5759/