Proposal: Add hostname `docker info`

Small proposal :dancer: 

In the multi-machines setup, it could be nice to be able to identify each host.

We propose to add a `--hostname` flag to the daemon and `Hostname:` in `docker info`

By default we use the system's hostname
Comma after (`MemTotal`). We should also probably specify what we mean by "hostname" here (i.e., FQDN or shortname or...).