'buildbot-syncer.js' should be able to determine force build argument from a list...

'buildbot-syncer.js' should be able to determine force build argument from a list of possible repositories.
https://bugs.webkit.org/show_bug.cgi?id=167152

Reviewed by Ryosuke Niwa.

Add 'rootOptions' key which maps to a list of possible repositories.
For a build request, only one of the repositories in the list is valid.

* tools/js/buildbot-syncer.js:
(BuildbotSyncer.prototype._propertiesForBuildRequest):
(BuildbotSyncer._validateAndMergeProperties):
(BuildbotSyncer):
* unit-tests/buildbot-syncer-tests.js:
(sampleiOSConfig):
(sampleiOSConfigWithExpansions):
(createSampleBuildRequest):
(Promise.resolve.then):
* unit-tests/resources/mock-v3-models.js:
(MockModels.inject):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@210909 268f45cc-cd09-0410-ab3c-d52691b4dbfc