[JENKINS-21073] Fix forceLdaps system property

https://issues.jenkins-ci.org/browse/JENKINS-21073

The [active directory plugin documentation](https://wiki.jenkins-ci.org/display/JENKINS/Active+Directory+plugin#ActiveDirectoryplugin-SecuringaccesstoActiveDirectoryservers) suggests setting `hudson.plugins.active_directory.ActiveDirectorySecurityRealm.forceLdaps` to force connecting via LDAPS.  This fixes two bugs:

* Updates the code to use the correct system property
* Changes the default LDAPS port from 686 to 636 (see http://support.microsoft.com/kb/321051/en-us)

I understand that STARTTLS is the preferred method, but that isn't working for me (not sure why).  LDAPS works, and this makes it a little easier to use.