allow LDAP-client integration for the OS user lookup

[FRosner]

Problem

When specifying the user and group, one has to know the uid and gid (#11).

Solution

Allow a simple LDAP lookup. Make LDAP server, dc configurable as environment variables. It should be compatible with #11 somehow.

Search for user

ldapsearch -x -LLL -H ldaps://<ldapserver>:636/ -b "ou=people,dc=<dc-subdomain>,dc=<dc-name>,dc=<dc-topleveldomain>" '(uid=<uid>)'

Search for group

ldapsearch -x -LLL -H ldaps://<ldapserver-socket>/ -b "ou=groups,dc=<dc-subdomain>,dc=<dc-name>,dc=<dc-topleveldomain>" '(cn=<cn>)'

[FRosner]

We'll do it without ldap search just by mounting the config probably.

Branch: ldap