[Merge] ~ddstreet/livecd-rootfs:remove-software-properties-dep into livecd-rootfs:ubuntu/master

[Dan Streetman]

The python3-software-properties package now depends on python3-launchpadlib, which isn't available for i386 (due to some of its arch-specific deps). So now python3-software-properties isn't available for i386, and its migration is blocked by livecd-rootfs's dependency on it, since livecd-rootfs does build for i386.

This is the only usage I saw in livecd-rootfs.
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https://code.launchpad.net/~ddstreet/livecd-rootfs/+git/livecd-rootfs/+merge/389235
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