Fix function to work with a node with multiple types of gpus

i.e.

gres=gpu:k80:4,gpu:tesla:4

the code would put the k80 as one number 4, and then add the tesla and
not remove the 4 but add the 8 so if all your other nodes only have 1
gpu and you are asking for 2 nodes it would return 4 instead of 1.

Bug 7066