basic ldap mocking

LDAP_SERVICE_MOCK must be set to True to enable ldap mocking. As a
security measure, mocking is only allowed in debug mode. Also changed
ACL_ADMIN_GROUP and ACL_SELFSERVICE_GROUP in default_config.cfg to more
distinguishable values and removed the unused ACL_LDAP_GROUP_USEREDIT
entry.

The sample entries in ldap_server_entries.json are the bare minimum
required for mocking and most likely won't suffice on a real LDAP server.
Also ldap3's mocked server is very limited and does not correctly verify
hashed password, update group memberships or implement ldap
overlays/extentions correctly.

Closes #20