Add ldap support

[Clemenk]

Adding this so we can use ldap as a backend for users.
Without this, php ldap functions cannot be found.

[Clemenk]

A TLS (ldaps or starttls) couldn't be made because php wants to validate the server's cert.
Commit 282271f add the /etc/ldap/ldap.conf file that php uses to get ldap configuration.

I chose to trust the CA that are present in /etc/ssl/certs. One could also add the TLS_REQCERT never option to tell the ldap client (php in our case) to not request or check server cert.