upgrades: add step to migrate tools to environment storage
Address review comments
upgrades: update toolstorage migration with tools-stream support
cmd/jujud: fix upgrade-steps test
Merge pull request #796 from axw/upgrade-toolstorage

upgrades: add step to migrate tools to environment storage

Tools are now stored in environment storage; we need to
migrate existing ones across from provider storage.
A followup will remove provider storage from the tools
search path. I expect there to be a lot of noise due to
tests relying on the tools being there.