Add ~amd64 ~x86 to missing KEYWORDS for mir-base/mir and mir-base/unity-system-compositor
Add 'cloud' USE to unity-base/unity-meta to enable/disable UbuntuOne cloud storage (enabled by default), and re-order some ubuntuone deps. so that unity-base/ubuntuone-control-panel sits at the top of the deptree pulling in ubuntuone-client and ubuntuone-client pulls in ubuntuone-client-data