Create utility to concatenate linux style paths to be evaluated on node

[jafreck]

Paths that are created on the client, but evaluated on the node need to be in linux style. That is currently done ad-hoc and leads to errors with double slashes occasionally:

# constants.py
SOME_DIR='wd/'
SOME_FILE='file.txt'

# function.py
import constants
path = constants.SOME_DIR + "/" + constants.SOME_FILE

A utility function would help this.