[GCU] Ignore bgpraw table in GCU operation (sonic-net#2628)

What I did
After the previous fix sonic-net#2623 , GCU still fails in the rollback operation. The bgpraw table should be discard in all GCU operation.
Thus, I change get_config_db_as_json function to crop out "bgpraw" table.

How I did it
Pop "bgpraw" table if exists.

How to verify it
Unittest

generic_config_updater/change_applier.py

@@ -76,7 +76,6 @@ class ChangeApplier:
     def __init__(self):
         self.config_db = get_config_db()
         self.backend_tables = [
-            "bgpraw",
             "BUFFER_PG",
             "BUFFER_PROFILE",
             "FLEX_COUNTER_TABLE"

generic_config_updater/gu_common.py

@@ -54,7 +54,9 @@ def __init__(self, yang_dir = YANG_DIR):
 
     def get_config_db_as_json(self):
         text = self._get_config_db_as_text()
-        return json.loads(text)
+        config_db_json = json.loads(text)
+        config_db_json.pop("bgpraw", None)
+        return config_db_json
 
     def _get_config_db_as_text(self):
         # TODO: Getting configs from CLI is very slow, need to get it from sonic-cffgen directly

tests/generic_config_updater/gu_common_test.py

@@ -3,12 +3,24 @@
 import jsonpatch
 import sonic_yang
 import unittest
-from unittest.mock import MagicMock, Mock
+from unittest.mock import MagicMock, Mock, patch
 
 from .gutest_helpers import create_side_effect_dict, Files
 import generic_config_updater.gu_common as gu_common
 
 class TestDryRunConfigWrapper(unittest.TestCase):
+    @patch('generic_config_updater.gu_common.subprocess.Popen')
+    def test_get_config_db_as_json(self, mock_popen):
+        config_wrapper = gu_common.DryRunConfigWrapper()
+        mock_proc = MagicMock()
+        mock_proc.communicate = MagicMock(
+            return_value=('{"PORT": {}, "bgpraw": ""}', None))
+        mock_proc.returncode = 0
+        mock_popen.return_value = mock_proc
+        actual = config_wrapper.get_config_db_as_json()
+        expected = {"PORT": {}}
+        self.assertDictEqual(actual, expected)
+
     def test_get_config_db_as_json__returns_imitated_config_db(self):
         # Arrange
         config_wrapper = gu_common.DryRunConfigWrapper(Files.CONFIG_DB_AS_JSON)