juju ensure-availability should default series to the current API series

[John A Meinel]

I'm splitting this part out of bug #1308966

For "juju ensure-availability" it makes the most sense that instead of just using "default-series" from the environment (especially if nothing is set), that we would actually just mirror the series for the existing API servers.

In fact, I would actually pick that value over default-series. Since default-series is a bit more about the charms that are being deployed.