Adding options for setting environment variables used by Condor

Previously all Condor submit files used the getenv = True option. This feature is being phased out, in favor of the more explict environment = "key1='value1' key2='value2' ...".

This commit adds a new config option in the [General] section: environment_mode. We default to environment_mode = getenv, which reproduces the previously hard-coded behavior (so there is no backwards incompatibility). We also support the option environment_mode = environment, which explicitly passes the items in os.environ to the environment option in Condor.

Fixes #131 (closed). Depends on rapidpe!52 (merged).