Sort names by number

Fixes bug #1369794: if machine names are numbers then sort them as numbers.
I suspect that new10 would be placed between new1 and new2. The same for 1/lxc0, 10/lxc0, 2/lxc/0.
I don't have a strong opinion about handling those cases too, but perhaps we should document this behavior by including those kind of machine names to tests.