Major refactoring of GRES reconfig/update logic

Add new logic to set node's GRES string on reconfig.
Set node GRES sockets based upon real socket/core config info.
Validate GRES changes before making them.
    Specifically we want to make sure no requests to process changes
    in the count of a GRES associated with File specifications happens.
    For example, if we have 4 gres/gpu associated with /dev/nvidia[0-3]
    and "scontrol update NodeName=... Gres=gpu:2" is executed that
    request will return an error. This is because we have no idea which
    specific gres/gpu records should be removed. We can issue a request
    to keep the count unchanged or set the count to 0, but any other
    count will return an error. This restrictions is not placed on
    GRES without Files (e.g. gres/craynetwork).
Changed some variable names to better reflect their contents.