Switch to python shell

Currently xenv uses $LMOD_CMD bash and parses the output. That creates problem for some escaped strings:

$LMOD_CMD bash load GCC OpenMPI | grep "OMPI_MCA_btl="
OMPI_MCA_btl=\^uct,openib;

We should switch the logic to use $LMOD_CMD python, replace all os.environ references by our own dictionary, and run the entire code through eval().