Add ability to force version bumps?

[jku]

Currently I believe there is no way to say "I'd like to resign a new version now instead of waiting for the next signing period to start". This is relevant at least in the case where targets and root have the same signers: In that case if you are doing an update on one you could as well sign the other (to avoid a separate signing event in near future).

The easy path is likely just changing tuf-on-ci-delegate so that there is a third top-level choice:

Configuring role root
 1. Configure signers: [@-repo-import, @jku], requiring 1 signatures
 2. Configure expiry: Role expires in 3650 days, re-signing starts 365 days before expiry
 3. Resign without changes

[jku]

this is basically a duplicate of #230: the consensus so far is that this (forcing a version bump) seems to be not needed.