* [RFC] Pass pytest options globally.

In many places having a global pytest flag is useful . For me with the
build and test of tvm , I would like to be able to globally pass in
pytest options as part of development flow or CI flows where one would
like to measure other things regularly that need measurements including
pytest coverage data that I would like to experiment with across the stack.

This has been achieved with an additional setup-pytest-env.sh file in
tests/scripts rather than putting in something in every single task test
script and something I would like to avoid.

This now means the -v option to pytest is superfluous. I did consider
having a pytest.ini file but that doesn't allow me to pass any old
environment variable in and this seems to be the compromise.

* Improve other use case documentation

* Rationalize pytest environment.

* Remove the setting from docker/with_same_user.
* Take the opportunity to migrate common PYTHONPATH and
TVM_PATH into the common environment setting.

* Fixup vta fsim

* Be more explicit with common PYTHONPATH

* Fix python path for task_python_vta_fsim.sh properly

* Fix nit in documentation.