support using environment variable in job template (for command args)

For example:

in job template: command.py -i ${QIIME_SOFTWARE}/otus.fasta
at client side: ${QIIME_SOFTWARE}=/home/ubuntu/data

after workunit checkout to the client:
the command will be parsed to:
command.py -i /home/ubuntu/data/otus.fasta

note: the availability of env variable will rely on client env