gplazma-ldap: make DN search string configurable

added a new property to specify DN search criteria:

gplazma.ldap.userfilter

The default value is 'uid', which is used in posix
schema.

This is a very lite filter version. Can be changed if needed.

Acked-by: Paul Millar
Target: master,2.6
Require-book: yes
Require-notes: yes
Patch: http://rb.dcache.org/r/5922/
Merge pull request #157 from kofemann/flex-ldap-2.6

flexible  DN query in ldap plugin