Blender Git Loki

Metadata: add hostname to the available metadata options

Having the hostname allows us to identify which machine rendered which
frame in our render farm.

This simply uses the host's name, and doesn't do any DNS lookup of any
IP address of the machine. As such, it's only usable for identification
purposes, and not for reachability over a network.

Reviewers: sergey, brecht

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D4047