reprepro: add i386 to trusty

While we don't expect to ever install an i386 server anymore, trusty
comes with multiarch enabled by default and it complains if it doesn't
find it. Rather than use deb [arch=amd64] all over the place, create an
empty i386 repository instead.

Change-Id: I1ac4d4862af14d92555b24b0be61339462e0b6a4