gplazma: ldap plugin: recreate dir context

In some environments the connection between gPlazma and the ldap
server may be closed by an existing firewall (e.g., at DESY).
This patch changes the DirContext from being final to a newly
created one on every call.

Acked-by:
Target: 2.6
Request: 2.6-cloud
Require-book: no
Require-notes: no