legacy-leases feature flag causes upgrade-series worker to fail to start

[John A Meinel]

If you either bootstrap with "--config features=[legacy-leases]" or use "juju controller-config" to turn it on you start to get this error:
2018-12-13 11:02:20 ERROR juju.worker.dependency engine.go:632 "upgrade-series" manifold worker returned unexpected error: unable to get leadership pinner; pinning is not available with the legacy lease manager
(this gets repeated every 3 seconds as the worker is restarted).

Digging into it, it seems that the recent changes to handle old Leadership leases not supporting pinning interacts poorly with how the UpgradeSeries facade is designed. It embeds the Leadership pinning api, and so it fails during New* rather than just saying "pinning not supported" later on.

Ideally, we would just treat pinning as a no-op, but still let you "juju upgrade-series" even if legacy leases are enabled.

[Heather Lanigan (hmlanigan)]

https://github.com/juju/juju/pull/9576