[Storage] file_mounts with bucket url for file will download to a directory

[Michaelvll]

The following yaml will create a directory and download the file to the folder /s3-data-file/frame0.xtc, instead of /s3-data-file to be a file itself.

file_mounts:
  /s3-data-file: s3://fah-public-data-covid19-cryptic-pockets/human/il6/PROJ14534/RUN999/CLONE0/results0/frame0.xtc

This also causes the problem for the spot user:

It seems that mounting individual files may not be working properly in managed spot mode:

warning: unable to access '/home/ubuntu/.gitconfig': Is a directory

it worked fine in on-demand mode
The yaml config is like:

file_mounts:
  ~/.gitconfig: ~/.gitconfig