version: add OSSupportedSeries

OSSupportedSeries returns the supported series
for the specified OS.
Unexpose environs/tools.ToolsLtsSeries
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.

A separate commit will change the API server
side to disregard the parameter, and compute
the supported series on the server side. We
can deprecate the client side parameter in a
future version.

Also: return a generic error if status code
is not 200. We no longer need to check the
"method not supported" error specifically.
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.
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/api: use juju/errors
Merge pull request #613 from axw/cmd-juju-upgradejuju-series

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.