Various cleanups in upgrade-juju

**cmd/juju: remove redundant code from upgrade-juju**

- Ignore/obsolete "--series" as in "juju bootstrap". UploadTools
  now computes the series to send to the API server (for backwards
  compat), and the API server now computes the same.
- Remove old backwards compat code.

Also, change jujud/bootstrap to use new version.OSSupportedSeries.


**state/apiserver: clone uploaded tools for supported series**

Disregard additional series specified by client, and clone for all
supported series of the same OS as the uploaded tools.

**state/api: always "upload" all supported series**

UploadTools no longer accepts a "fakeSeries" parameter; the client just
sends all of the supported series of the same OS as the uploaded tools.
This is to support old API servers.

Also: return a generic error if status code is not 200. We no longer need
to check the "method not supported" error specifically.

**environs/tools: unexpose environs/tools.ToolsLtsSeries**

**version: add OSSupportedSeries**

OSSupportedSeries returns the supported series for the specified OS.
Any Ubuntu series would do, we don't need to test all of them.