sanity check fixes for chassis; and cache cleanup at the end of pytest session (#2958)

Approach
What is the motivation for this PR?
sanity check:

The BGP and interface sanity checks for multi-dut were changed to iterate through all the nodes in a multi-dut setup. However, for a T2 chassis, the multi-dut testbed contains a supervisor card as well, which doesn't have any BGP/PORT configuration. Thus, these checks fail for supervisor card.
cache cleanup:

If we change the inventory file and re-run pytest, the old cached data for the inventory is used instead of picking up the changes.

How did you do it?
sanity_check:
Iterate through frontend_nodes of duthosts instead of all the nodes.
cache cleanup:
Added fixture cleanup_cache_for_session that is called at the beginning of a session to remove the cached facts for all the DUTs in the testbed. This is not an automatic fixture, and is needed in the following scenarios:
Running tests where some 'facts' about the DUT that get cached are changed.
Running tests/regression without running test_pretest which has a test to clean up cache (PR#2978)
Test case development phase to work out testbed information changes.

Author: sanmalho-git

tests/common/plugins/sanity_check/checks.py

@@ -106,7 +106,7 @@ def _find_down_ports(dut, phy_interfaces, ip_interfaces):
 def check_interfaces(duthosts):
     def _check():
         check_results = []
-        for dut in duthosts:
+        for dut in duthosts.frontend_nodes:
             logger.info("Checking interfaces status on %s..." % dut.hostname)
 
             networking_uptime = dut.get_networking_uptime().seconds
@@ -161,7 +161,7 @@ def _check():
 def check_bgp(duthosts):
     def _check():
         check_results = []
-        for dut in duthosts:
+        for dut in duthosts.frontend_nodes:
             def _check_bgp_status_helper():
                 asic_check_results = []
                 bgp_facts = dut.bgp_facts(asic_index='all')

tests/conftest.py

@@ -880,3 +880,19 @@ def duthost_console(localhost, creds, request):
                        console_password=creds['console_password'][vars['console_type']])
     yield host
     host.disconnect()
+
+@pytest.fixture(scope='session')
+def cleanup_cache_for_session(request):
+    """
+    This fixture allows developers to cleanup the cached data for all DUTs in the testbed before test.
+    Use cases:
+      - Running tests where some 'facts' about the DUT that get cached are changed.
+      - Running tests/regression without running test_pretest which has a test to clean up cache (PR#2978)
+      - Test case development phase to work out testbed information changes.
+
+    This fixture is not automatically applied, if you want to use it, you have to add a call to it in your tests.
+    """
+    tbname, tbinfo = get_tbinfo(request)
+    cache.cleanup(zone=tbname)
+    for a_dut in tbinfo['duts']:
+        cache.cleanup(zone=a_dut)