Auto merge of #659 - aneeshusa:enable-choosing-machine-for-force-builds, r=edunham
Auto merge of #659 - aneeshusa:enable-choosing-machine-for-force-builds, r=edunham

Enable choosing build machine for force builds

This makes it easier to test if changes to a specific build machine are
working properly, e.g. the current impetus of seeing if Android-related
changes on servo-linux-cross3 are working.

Docs: https://docs.buildbot.net/0.8.12/manual/cfg-schedulers.html#buildslavechoiceparameter

Fixes: #658.

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servo/saltfs,servo/saltfs