Instaslice should select the container that needs GPU access

[harche]

As of now, instaslice only supports one container per pod to request GPU access. However, from the code it appears that this one container also needs to be the first container in the list of containers in the pod. This seems rather restrictive and unnecessary. Instaslice should be able to go over the list of containers of the pod and select the one that is requesting the GPU access irrespective of the order it is declared in the pod definition.

/cc @rphillips

[asm582]

this issue is related to #25