Add environment variables for submit, log, out, and error directories

[jrbourbeau]

Description

It would be nice if pycondor supported setting environment variables for Job and Dagman submit, log, out, and error directories. So something like

>>> import pycondor
>>> job = pycondor.Job('myjob', '/path/to/executable.py`)
>>> job.build()

would automatically check for environment variables like

PYCONDOR_SUBMIT_DIR=/path/to/submit
PYCONDOR_LOG_DIR=/path/to/log
PYCONDOR_OUT_DIR=/path/to/out
PYCONDOR_ERROR_DIR=/path/to/error

and set the directories in the built submit file appropriately. E.g.

# submit file at /path/to/submit/myjob.submit
output = /path/to/out/myjob.out   
error = /path/to/error/myjob.error  
log = /path/to/log/myjob.log