Optionally disable Python environment

pydoop defaults to copying the python executable path and environment variables from the client. In general this won't work when running jobs on a remote Hadoop cluster, this PR adds a `--no-override-env` so that the map-reduce script will use the default python executable and won't set any environment variables.

A longer term solution would be to allow a custom environment to be configured, e.g. as command line arguments or by passing an environment file.