Fix model and owner slug validation (#1134)

Validate model slug and owner slug in model-metadata.json before sending
to API.
Model slugs must be lowercase, alphanumeric, and contain only hyphens.
Owner slugs may also contain underscores.

Fixes: #1133

TAG=agy
CONV=7b917f8b-ff11-48c4-a39a-aaf134a39ca6