Introduce and use FindTools provisioner API

We introduce a new FindTools API in the
provisioner facade, and move the existing
client FindTools API implementation into
the apiserver/common package. The provisioner
worker is updated to use this API instead
of going directly to provider storage (via
environs/tools.FindTools).

There are a couple of bugs already that require
this to be done, but now that we're moving tools
storage behind the API server it is imperative.

Fixes https://bugs.launchpad.net/bugs/1347984
Fixes https://bugs.launchpad.net/bugs/1304151
Merge pull request #636 from axw/provisioner-tools-api

Introduce and use FindTools provisioner API

We introduce a new FindTools API in the
provisioner facade, and move the existing
client FindTools API implementation into
the apiserver/common package. The provisioner
worker is updated to use this API instead
of going directly to provider storage (via
environs/tools.FindTools).

There are a couple of bugs already that require
this to be done, but now that we're moving tools
storage behind the API server it is imperative.

Fixes https://bugs.launchpad.net/bugs/1347984
Fixes https://bugs.launchpad.net/bugs/1304151