Fix Z motor mount position so motor sits centered vs mounts

[triffid]

I found that my motors don't sit centered in the mounts- the mounting holes are ~1mm too close to the Z rod clamp. First commit fixes this.

Second commit shrinks the flange hole slightly (motor still fits) and adds some M3 screw holes so I can mount a 608 bearing instead of a motor. It shouldn't compromise the bracket or alter its existing function in any way that I can imagine, while providing a potential upgrade path for those of us who have 4 motors and can't get a 5th just yet.

[droftarts]

See Issue 38 https://github.com/prusajr/PrusaMendel/issues/38 I think you have made the Z-leadscrew run even more eccentrically!