Allow specifying custom Ubuntu archive mirrors in build

[林博仁(Buo-ren Lin)]

Currently, snapcraft uses archive.ubuntu.com for the software archive and it's not optimal, please support a mechanism to do so.

## Workaround ##
Patch the source list after the build is initialized(would have to repeat it after every `snapcraft clean`)

### Multipass build ###
```
multipass exec snapcraft-my-snap-name -- sudo sed -i 's@//archive.ubuntu.com@//my.fovourite.archive.mirror@' /etc/apt/sources.list.d/default.sources
multipass exec snapcraft-my-snap-name -- sudo apt update
```

### LXD build ###
```
lxc exec snapcraft-my-snap-name -- sed -i 's@//archive.ubuntu.com@//my.fovourite.archive.mirror@' /etc/apt/sources.list.d/default.sources
lxc exec snapcraft-my-snap-name -- apt update
```

[Alan Pope 🍺🐧🐱 🦄 (popey)]

I too would appreciate being able to use a custom mirror. However I'd rather not use --enable-geoip, but something which lets snapcraft use a specific mirror. For example I have a mirror at home on a specific host/IP which I'd prefer to use, to reduce my bandwidth usage, and speed up builds.

[林博仁(Buo-ren Lin) (brlin)]

@popey
Agreed, description fixed.

[Iain Lane (laney)]

Hey, I just came here to file / look for this bug too.

I'm thinking, though, that this one is a duplicate of bug #1602085 so I'll mark it so (and copy your workaround there).

Thanks :-)