If connect_share_server_to_tenant_network is on, default route for service subnet isn't created

## Description

Hello.
the problem is that the created instance has 2 interfaces
(Manila-service-network and tennant network)
The service image handles this in such a way that the default gateway attaches
to a random interface.

if user want to assign floating IP to the tennant network interface this
may not work, since the gateway will be in the service network.

## Steps to reproduce

* Set configuration:
  * driver_handles_share_servers = True
  * connect_share_server_to_tenant_network = True

* Create share.
* Check route map on manila instance in service project.

## Environment

* Manila 14.0.0
* Neutron network