pre-define allowed authentication method via config property

If several authentications methods are installed, one might still want to limit the list of available methods to e.g. just LDAP or O-Auth etc. Therefore a new config property restrict_auth_methods should be introduced, with a list of possible params: ldap,basic,oauth2,etc. If list is empty all installed options should be available. List must also allow for mutli-values, e.g ldap and basic. (relates to #36 (closed))