Make cisco_csr_identifier migration conditional

neutron dropped 'cisco_csr_identifier_map' table in
[1], this makes migrations for that table conditional.

[1] https://review.opendev.org/c/openstack/neutron/+/897472
Related-Bug: #2038555

Change-Id: I31f2175cfc91a1d2bcb15b8911b2e323af25740d

neutron_vpnaas/db/migration/alembic_migrations/versions/liberty/contract/2c82e782d734_drop_tenant_id_in_cisco_csr_identifier_.py

@@ -22,6 +22,7 @@ Create Date: 2015-08-20 15:17:09.897944
 """
 
 from alembic import op
+import sqlalchemy as sa
 
 from neutron.db import migration
 
@@ -35,4 +36,7 @@ neutron_milestone = [migration.LIBERTY]
 
 
 def upgrade():
+    insp = sa.inspect(op.get_bind())
+    if 'cisco_csr_identifier_map' not in insp.get_table_names():
+        return
     op.drop_column('cisco_csr_identifier_map', 'tenant_id')

neutron_vpnaas/db/migration/alembic_migrations/versions/liberty/contract/333dfd6afaa2_populate_vpn_service_table_fields.py

@@ -58,6 +58,9 @@ def _migrate_external_ips(engine):
     addresses for an IP version, then only the first one will
     be stored (the same as the reference driver does).
     """
+    insp = sa.inspect(engine)
+    if 'cisco_csr_identifier_map' not in insp.get_table_names():
+        return
     session = sa.orm.Session(bind=engine.connect())
     services = session.query(VPNService).all()
     for service in services:

neutron_vpnaas/db/migration/alembic_migrations/versions/liberty/contract/56893333aa52_fix_identifier_map_fk.py

@@ -38,6 +38,9 @@ branch_labels = (cli.CONTRACT_BRANCH,)
 
 
 def upgrade():
+    insp = sa.inspect(op.get_bind())
+    if 'cisco_csr_identifier_map' not in insp.get_table_names():
+        return
     # re-size existing data if necessary
     identifier_map = table('cisco_csr_identifier_map',
                            column('ipsec_site_conn_id', sa.String(36)))

neutron_vpnaas/db/migration/alembic_migrations/versions/rocky/contract/e50641731f1a_drop_cisco_csr_identifier_map_table.py

@@ -22,6 +22,8 @@ Create Date: 2018-02-28 10:28:59.846652
 """
 
 from alembic import op
+import sqlalchemy as sa
+
 from neutron.db import migration
 
 # revision identifiers, used by Alembic.
@@ -34,4 +36,7 @@ neutron_milestone = [migration.ROCKY, migration.STEIN,
 
 
 def upgrade():
+    insp = sa.inspect(op.get_bind())
+    if 'cisco_csr_identifier_map' not in insp.get_table_names():
+        return
     op.drop_table('cisco_csr_identifier_map')