When dst network is created (not found), the returned dict does not have
subnets attribute initialized. That may cause issues in later stages,
where we rely on at least one `subnet_id` being present (see
`describe_server_network_connection` function).

Fixed by getting the network from OStack again, after everything has
been created.

See failed pipeline:
- Dst network without subnet
  - https://gitlab.ics.muni.cz/cloud/g1-g2-ostack-cloud-migration/-/jobs/1604037#L72
- Error later in execution
  - https://gitlab.ics.muni.cz/cloud/g1-g2-ostack-cloud-migration/-/jobs/1604037#L146
- Final assertion error
  - https://gitlab.ics.muni.cz/cloud/g1-g2-ostack-cloud-migration/-/jobs/1604037#L164