[Merge] ~robert-ancell/software-properties:ubuntu-pro-merge into software-properties:ubuntu/master

[Robert Ancell]

This is in the final stages of review, so worth proposing now for code review purposes. We shouldn't land until it's confirmed as correct.

Available for testing in https://launchpad.net/~robert-ancell/+archive/ubuntu/ubuntu-pro
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https://code.launchpad.net/~robert-ancell/software-properties/+git/software-properties/+merge/430260
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~robert-ancell/software-properties:ubuntu-pro-merge into software-properties:ubuntu/master.