Support S3 paths in h2o.load_grid and h2o.loadGrid

[hutch3232]

Is your feature request related to a problem? Please describe.
h2o's python package supports S3 paths in h2o.save_grid yet h2o.load_grid throws a "Not Implemented Error". The same is true for the corresponding R package functions. It seems like you should be able to save/load from the same paths like you can do for non-grid model objects.

Describe the solution you'd like
Implement S3 path support for h2o.load_grid and h2o.loadGrid.

Describe alternatives you've considered
Manually copy the grid files to a temporary file storage then apply h2o.load_grid.