[voq] Add fabric_ns to the ns_list when display_option is DISPLAY_ALL. (

#2717)

The get_ns_list_based_on_options function only gets front_ns and back_ns in current code.
This change adds fabric_ns in the ns_list when display_option is DISPLAY_ALL. This enables the fabric related tests on voq chassis, e.g the change in sonic-net/sonic-mgmt#6620

utilities_common/multi_asic.py

@@ -63,12 +63,13 @@ def get_ns_list_based_on_options(self):
             namespaces = multi_asic.get_all_namespaces()
             if self.namespace_option is None:
                 if self.get_display_option() == constants.DISPLAY_ALL:
-                    ns_list = namespaces['front_ns'] + namespaces['back_ns']
+                    ns_list = namespaces['front_ns'] + namespaces['back_ns'] + namespaces['fabric_ns']
                 else:
                     ns_list = namespaces['front_ns']
             else:
                 if self.namespace_option not in namespaces['front_ns'] and \
-                        self.namespace_option not in namespaces['back_ns']:
+                        self.namespace_option not in namespaces['back_ns'] and \
+                        self.namespace_option not in namespaces['fabric_ns']:
                     raise ValueError(
                         'Unknown Namespace {}'.format(self.namespace_option))
                 ns_list = [self.namespace_option]