CBD-1249:  platform is one of: centos-6-x64, ubuntu-1204-x64, windows-x64

Change-Id: I70e29c1de99858062f7092acf5a0036c60b98867
Reviewed-on: http://review.couchbase.org/39680
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>