gplazma2-htpasswd: Avoid deprecated Guava classes

The plugin was incompatible with Guava 18 due to use of deprecated
classes that are removed in the new release.

This patch ports the plugin to Java 8.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7465/