tests: Start the ability to mark tests

tests: Start the ability to mark tests

Add the ability for our topotests to take advantage of pytest `mark`ing.
This effectively allows you to tell pytest to run against certain sets
of tests.  For a demonstration purpose I've added in marks for:
babel
eigrp
ldp
ospf
pim
rip

And setup tests to run against those tests that only test those protocols.

You can run against eigrp tests by running `pytest -k eigrp`
Other combinations are also available based upon simple boolean logic.
Just read the pytest.mark documentation.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>