tests: change conftest.py to support dynamic markers and test selection

tests: change conftest.py to support dynamic markers and test selection

This applies correct markers to tests depending on their file path.
Tests that live at the root of tests/functional/tests will be given a
marker of 'all' and apply to any type of ceph node.

Tests that live in tests/functional/tests/mon will get the
'mons' marker and will only run for nodes in the 'mons' group.

Tests that live in tests/functional/tests/osd will get the 'osds'
marker and will only run for nodes in the 'osds' group.

All tests must use the CephNode fixture for this to work and be
parameterized by testinfra.

Signed-off-by: Andrew Schoen <aschoen@redhat.com>