Data paths should be tied to CLI for Resnet app

[benson31]

The resnet.py app is too restrictive about its data paths. In particular, if a user at LC without lbann group access or at another institution wants to run it, they must modify Python code. Normally I'd say this is fine (it's only Python), but unfortunately for us, there's now precedent -- at the very least, the Cosmoflow app allows data paths to be overridden on the command line. So we should make this more pervasive.