22001: Simplify virtualenv management in run-tests

22001: Simplify virtualenv management in run-tests

A lot of the current virtualenv management code is parameterized,
probably to able to support virtualenvs for Python 2 and 3 in
parallel. Since we don't need that anymore, remove all this code to
simplify the script.

* initialize() calls setup_virtualenv(), which ensures the virtualenv
  exists and then activates it. Everything else in the script can assume
  this is done.
* install_env() installs the Python build tools and support libraries
  we need.
* install_deps() installs our Python modules that are used by other
  components in Arvados.

With all this, there should be no need for regular virtualenv
de/reactivation.

Various old incantations meant to work around old setuptools/pip bugs
got cleaned up as part of this too.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>