[Merge] ~chad.smith/ubuntu-release-upgrader:mirrors/ua-services into ubuntu-release-upgrader:ubuntu/master

[Brian Murray]

Review: Approve

I've gone ahead and merged this change. One thing I discovered though was that the update-mirrors script (in utils) also needed updating as it has a count of the "permanent lines" in mirrors.cfg. So if there are any future additions to mirrors.cfg the permanent lines count should be incremented.
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https://code.launchpad.net/~chad.smith/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/390102
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