Download tools from API server

We will be eliminating the use of provider storage for tools. This is a
step in that direction: hiding the use of provider storage behind an HTTP
API provided by the API server for fetching tools. Later we will change the
backend to fetch the tools from GridFS, when that work has been
implemented.

Now that we are serving tools in the API server, we must consider the
availability of the API server at the time a machine is provisioned. The
aria2 package is an alternative to curl that is capable of failing over to
additional sources and optionally downloading chunks in parallel.
Note: curl can be built with support for Metalink, which would have been a
more conservative option. Unfortunately the version of curl packaged in
Ubuntu does not have Metalink support enabled.

The local provider always has tools available locally, and user-data for
lxc and kvm does not have the size limitations as in cloud providers. Thus,
we can safely serialise the tools tarball into user-data in the local
provider.
I'd rather put a limit on it so we can pull up stumps and report back a specific error to Juju after a reasonable number of attempts. Otherwise we're just propagating the pending forever behaviour.