gplazma2-ldap: use url rather than host+port

the ldap plugin builds the url of the ldap server using

"ldap://" + host + ":" + port

this way the protocol always is always "ldap". This prevents
the plugin from using secure ldap using "ldaps".
This patch substitutes host and port by a freely definable url.

Ticket: 8289
Acked-by:
Target: trunk
Require-book: yes
Require-notes: yes
remove deprecated annotation for back-port
Merge pull request #534 from kschwank/pull-req-ldap/2.8

Pull req ldap/2.8