Add OSG env variable support
NEW: "which" utility

Mimics the SHELL command "which"
Use the which utility

to find the glexec in the PATH (if it's there)
FIX: submission is not a string

The submission['Value'] is sometimes a tuple, in the glexec case anyway, so it needs a cast to string
Merge pull request #1143 from sposs/fix-glexec

Fix glexec