Change sync-tools to use API

environs/sync now operates on two interfaces:
 - a ToolsFinder, which is used for finding
   tools in the destination
 - a ToolsUploader, which is used for uploading
   missing tools to the destination

There are two implementations of ToolsFinder
and ToolsUploader: one using Storage, and one
that uses the client API.

In cmd/juju we now use the client API unless
--local-dir is specified. --public now only
takes effect if --local-dir is specified. This
is redundant anyway, given the metadata plugins.
I'd like to see these methods commented. They are named similarly to, but differ from, the ones on ToolsFinder and ToolsUploader