Fix the schema migration that adds the neutron-l3 role

The nodes that had the "neutron-server" role assigned before also need
"neutron-l3" after upgrading the schema. Otherwise they end up with an
incomplete run_list. (bnc#875022)
Merge pull request #86 from rhafer/fix-l3-migration

Fix the schema migration that adds the neutron-l3 role