Replace system test runner with pytest

The legacy solution for running systems test has evolved over the course of years and is currently a mix of shell & perl scripts intermingled with the build system, while some of the system tests utilize pytest. Implementing a more consistent solution using just pytest as a runner could bring following benefits:

• better test run isolation (i.e. artifacts from previous run don't interfere with current test run)
• more precise control over test selection (running just a single test case)
• getting rid of perl+shell glue scripts
• a simpler and more standard way to run and parallelize test runs
• solid foundation for future extensions (e.g. wrapping test execution inside a network/pid namespace)

For a transitory period of time, the legacy test framework should be supported, since it'd be difficult to replace everything at once. The pytest runner should be available in 9.18+, it'd be prudent to keep the legacy runner support until 9.16 reaches EOL. By that time, we should have enough insight to determine whether pytest proves to be a suitable replacement and throw away the legacy runner from supported branches at that point.

Migration plan for moving to pytest runner and dropping the legacy runner support:

• Phase I - pytest runner development, legacy runner supported
  • initial implementation of the pytest runner (#3978 (closed), !6809 (merged))
  • support out-of-tree tests (#4246 (closed))
  • resolve support on CI systems with old pytest (OpenBSD, CentOS 7) (!8193 (merged))
  • implement any missing (and desired) features from legacy runner (#4252 (closed))
  • configure make check to invoke pytest (#4262 (closed))
• Phase II - deprecating legacy runner - 9.19-only
  • remove legacy runner control script(s) - legacy.run.sh, get_ports.sh ... (#4251 (closed))
  • remove no longer needed scripts from system tests (e.g. clean.sh) (#4261 (closed))
  • remove conf.sh(.common) and declare variables in pytest only (!8800 (merged))
  • remove the Makefile entanglement
  • declare python and pytest-xdist as required dependencies for tests + document
  • address any FUTURE comments in the pytest runner code
• Phase III - cleanup after legacy runner
  • rewrite start.pl/stop.pl to python (related #3198 (closed))
  • rewrite remaining setup/teardown perl&shell scripts to python
  • rewrite setup.sh/prereq.sh system tests scripts to pytest fixtures
  • ensure system test documentation is up to date