Different python version for the az commands

[Mandur]

The DSVM has two python versions
One under /anaconda/envs/py35/bin/python which the standard command pip install relates to.
One under /opt/az/bin/python3 which the az command line related to.

so If users run pip install azure-cli-ml, this will install the ml command on the wrong python environment and users won't be able to run the az ml commands. This could currently by overcome by running sudo /opt/az/bin/python3 -m pip install azure-cli-ml but it would be less confusing to either provide an harmonized default python for both commands, or to describe this in the doc.