Fix `rollbackEtcdData()` to return error=nil on success
`rollbackEtcdData()` used to always return an error making the rest of the
upgrade code completely unreachable.

Ignore errors from `rollbackOldManifests()` during the rollback since it
always returns an error.
Success of the rollback is gated with etcd L7 healthchecks.

Remove logic implying the etcd manifest should be rolled back when
`upgradeComponent()` fails