New TurnKey OpenLDAP version (16.1)

Support running OpenLDAP appliance running as ununprivileged on LXC - closes #1535.
Use MDB backend. Previously we were using deprecated HDB backend.
Install latest upstream release of phpLDAPadmin from GitHub - v1.2.6.2. We were previously installing from 'master', but that is now tracking v2 development (no v2.x release yet).
Include Webmin LDAP module by default. Closes #864.
Note: Please refer to turnkey-core's 16.1 changelog for changes common to all appliances. Here we only describe changes specific to this appliance.