L2 gateway migration skip

Skip l2 gw migration if the table does not exist

Change-Id: Ib88e10e7c377d8dc1dece75fae1307174ff827cb

vmware_nsx/db/migration/alembic_migrations/versions/liberty/expand/279b70ac3ae8_nsxv3_add_l2gwconnection_table.py

@@ -30,14 +30,19 @@ import sqlalchemy as sa
 
 
 def upgrade():
-    op.create_table(
-        'nsx_l2gw_connection_mappings',
-        sa.Column('connection_id', sa.String(length=36), nullable=False),
-        sa.Column('port_id', sa.String(length=36), nullable=False),
-        sa.Column('bridge_endpoint_id', sa.String(length=36), nullable=False),
-        sa.ForeignKeyConstraint(['connection_id'],
-                                ['l2gatewayconnections.id'],
-                                ondelete='CASCADE'),
-        sa.ForeignKeyConstraint(['port_id'], ['ports.id'], ondelete='CASCADE'),
-        sa.PrimaryKeyConstraint('connection_id'),
-    )
+    try:
+        op.create_table(
+            'nsx_l2gw_connection_mappings',
+            sa.Column('connection_id', sa.String(length=36), nullable=False),
+            sa.Column('port_id', sa.String(length=36), nullable=False),
+            sa.Column('bridge_endpoint_id', sa.String(length=36),
+                      nullable=False),
+            sa.ForeignKeyConstraint(['connection_id'],
+                                    ['l2gatewayconnections.id'],
+                                    ondelete='CASCADE'),
+            sa.ForeignKeyConstraint(['port_id'], ['ports.id'],
+                                    ondelete='CASCADE'),
+            sa.PrimaryKeyConstraint('connection_id'),
+        )
+    except Exception:
+        pass