Remote execution client

Description

Remote execution client for BuildStream.

Changes proposed in this merge request:

• Add a new object derived from Sandbox which handles remote builds. This will look to BuildStream just like another Sandbox object, but will marshal all the necessary info up and send it to BuildGrid for execution.
• Add configuration option to allow remote execution to be used.

This merge request, when approved, will close issue/bug: #454 (closed)

There are a few things missing from this merge request at the moment:

• Finer detailed configuration options, as discussed at https://mail.gnome.org/archives/buildstream-list/2018-August/msg00066.html and replies
• Unit tests.

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