Make the name of the python interpreter configurable

[bwachter]

On my system the python 3 interpreter is called python3 - using a configurable variable instead of the currently hardcoded python in epc:start-epc would allow changing that without modifying the code.

[joshcho]

It should be addressed in 7e156fe3fd71b2f76fbc5f7de61be2dfd5a381fe