[r=fwereade],[bug=1234035] Fix upgrade 1.14 -> 1.15
This adds a Tools() API call to provisioner API,
similarly to the upgrader API. Common code factored
out into apiserver/common/tools.go.
SetAgentTools() renamed to SetAgentVersion() in
machine and unit state types. Also renamed a few
types in params to be shorter or better reflect
what are they about. Added DEPRECATE(v1.18) tags
to bits of the API that need cleanup.
Live tested on EC2 - after copying the tools from
/tools/releases/ to /tools/ the upgrade proceeds
and finishes successfully.
https://codereview.appspot.com/14231044/
R=fwereade, jameinel