Update metadata should say "variant" rather than "flavor"

[tjkirch]

#578 updated most of Thar to use "variant" rather than "flavor", but our update metadata affects end-user systems and so must be changed more carefully. This issue tracks updating the metadata to use the consistent "variant" terminology.

• Make updog check for "variant" first and fall back to "flavor"
• Update automation to write "variant", and add an option for duplicating the data in "flavor"
• When we transition to new repository URLs, stop adding "flavor"

Update steps:

• Change remaining flavor references to variant in updater and update_metadata
• Generate a new manifest that only contains the word `variant.

[tjkirch]

We're going to need a hard-break release for other reasons, so if we get this done soon, we wouldn't need the migration process described above.