Replace use of kpartx for ubuntu-core distrobuilder

[stgraber]

Because our production image building runs inside a container (privileged though), we can't use kpartx as it attempts to mess with device-mapper which is very very unpleasant to use inside containers.

We do however have working loop devices, so using losetup would be fine.

So we need to change distrobuilder to scan the partition table of the .img file, figure out the offset and size of the partition we want and then map that using losetup, then mount it.