Make it possible to change CPUsPerTask with scontrol.

This also reverts most of commit fa331e30 as well as commit bd9fa830
which would try to set the pn_min_cpus every time a job was updated.
If a job didn't request node counts then they were hosed.

This commit takes away the magic which was screwing things up.  Now the
person gets what they asked for without magic changing things.

Bug 2302
Bug 2742
Bug 2478