mgr/prometheus: add test cases and validation using tox

mgr/prometheus: add test cases and validation using tox

Focus all tests inside a tests directory, and use pytest/tox to
perform validation of the overall content. tox tests also use
promtool if available to provide rule checks and unittest runs.

In addition to these checks a validate_rules script provides the
format, and content checks against all rules - which is also
called via tox (but can be run independently too)

Signed-off-by: Paul Cuzner <pcuzner@redhat.com>