infra: Add and use mixins for Goma RBE server settings

R=tikuta@chromium.org, ukai@chromium.org, yyanagisawa@chromium.org

Bug: 945701
Change-Id: I98b2a8a2d644eccde84224171bbe7e9885041046
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1539416
Commit-Queue: Simon Que <sque@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644219}