Add hostname and ID docker info

replace #9126 

This PR add the Hostname to `docker info`; it's the machine hostname, so it might not be unique.
This PR also add the ID of the engine to `docker info`; this ID is generated using libTrust, it is unique.

I extracted some code from #8265
@vieux I agree we'd be better off describing what these mean. I suggest:
`a user-friendly name describing the running Docker daemon (<backtick>Name<backtick>), and a unique ID identifying the daemon (<backtick>ID<backtick>).` WDYT?