Support for environment variable for path to localimage

[nikkyjain]

When bootstrapping from localimage, it would be modular if user is allowed to specify environment variable. Using hard-coded paths in definition file should be avoided.

Example :

Bootstrap: localimage From: /grid/tfo/fast_fe_in_scratch/users/vishalj/sif/base.sif

It would be good if user could use environment variable here .. somewhat like this .. Bootstrap: localimage From: ${SIF_PATH}/base.sif