Tests: Refactor tearDown show command logging, add lifecycle markers.

This change adds a consistent interface for adding test-specific show commands to
log.txt.

It also adds log markers for the execution of setUp[Class], tearDown[Class]
in the logs.

Change-Id: I7d42e396e594a59e866a7d55dac0af25548e657a
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>

test/test_l2_fib.py

@@ -145,11 +145,13 @@ class TestL2fib(VppTestCase):
         """
         super(TestL2fib, self).tearDown()
         if not self.vpp_dead:
-            self.logger.info(self.vapi.ppcli("show l2fib verbose"))
             for bd_id in self.n_brs:
                 self.logger.info(self.vapi.ppcli("show bridge-domain %s detail"
                                                  % bd_id))
 
+    def show_commands_at_teardown(self):
+        self.logger.info(self.vapi.ppcli("show l2fib verbose"))
+
     def create_hosts(self, n_hosts_per_if, subnet):
         """
         Create required number of host MAC addresses and distribute them among