Engine: convenience http transport for simple remote job execution
Cleanup version introspection

* Unify version checking code into version.go
* Make 'version' available as a job in the engine
* Use simplified version checking code when setting user agent for registry client.
Port 'docker version' to the engine API
Fix #1229. Update titles, fix some wrapping.
Make the Ambassador container explicit.
Apply Sven's suggestions.
Merge pull request #3119 from shykes/engine-version

Port 'docker version' to the engine API
Merge pull request #3103 from metalivedev/1229-titleactions

Update "Use" titles to be action-oriented