[BuildStream] Proposal: Add 'remote-execution' project configuration

[Jim MacArthur <jim macarthur codethink co uk>]

I'd like to add an option to the project and element YAML spec to 
control remote execution. In the code, the remote execution part of 
BuildStream takes the form of a third sandbox, but I don't think it 
makes sense for the user to consider remote execution as a type of 
sandbox. Hence, I think this needs a new top-level label. If anyone 
thinks it'd fit into an existing top-level label I'd be happy to change 
it. Does this look like a reasonable approach?


Problem Statement

=================

We need a means to tell Buildstream when to use remote execution and the 
connection details for the build server.


Proposal

========

Add a 'remote-execution' top level label for project.conf and elements. 
This would contain a single element, 'url', containing the hostname and 
port. For example:

----

remote-execution:
  url: buildgridserver.example.com:50051

----

This should be usable in element configuration as well as project.conf, 
to enable or disable remote execution per-element. To disable remote 
execution in an element when it's enabled by project.config, 'url: ', 
'url: null' or 'url: ""' will be accepted.

We expect to add further options to specify SSL keys and certificates in 
the future and can raise a new proposal for that.


Implementation
==============

Very similar to current configuration options for the 'sandbox' label. 
The resulting value would be checked in element.__sandbox and used to 
select the remote sandbox object if present.

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