Do not set Nova tags longer than Nova accepts

Trove tags the Nova servers it creates with one tag per metadata entry, including trove_user_id_. Nova refuses tags longer than 60 characters and rejects the whole request, and Keystone ids of federated users are 64 characters long, so for those users the tag is 78 characters: every instance create fails at server boot with HTTP 400, and trove-manage set_nova_tags cannot tag their existing servers, which then disappear from the tags-any based instance listing and show up as SHUTDOWN.

Only the trove_instance_id_* and trove_project_id_* tags are read back, and the user id stays in the server metadata, so leave out any tag longer than 60 characters, with a warning, both when creating the server and in set_nova_tags.