added ldap_filter to ldap auth sources to filter login lookups

implements http://projects.theforeman.org/issues/863

You can now use filters like:

(&(memberOf=CN=ForemanUsers,OU=Groups,DC=some,DC=domain)(memberOf=CN=AdminUsers,OU=Groups,DC=some,DC=domain)) 

or simply memberOf=CN=ForemanUsers,OU=Groups,DC=some,DC=domain

Maybe an input valitdation using validates in combination with Net::LDAP::Filter.construct might me useful, too.