Use network service client to query source network

[votdev]

Problem:
During the preflight check the VM importer tries to make sure the specified spec.networkMapping[x].sourceNetwork in VirtualMachineImport exists for OpenStack imports.
The existing implementation seems not to work anymore with the latest OpenStack versions.

Solution:
Make use of the NewNetworkV2 ServiceClient to query for the specified source network.

Related Issue:
harvester/harvester#7432

Test plan:
Case 1
The test plan makes use of our internal OpenStack test environment.

• Create the Secret and OpenstackSource resources to be able to access the test environment.

apiVersion: migration.harvesterhci.io/v1beta1
kind: OpenstackSource
metadata:
  name: devstack
  namespace: default
spec:
  endpoint: "http://xxx.xxx.xxx.xxx/identity"
  region: "RegionX"
  credentials:
    name: devstack-credentials
    namespace: default

• Create the following manifest and apply it. Make sure the destination network default/vlan1 exists in Harvester.

apiVersion: migration.harvesterhci.io/v1beta1
kind: VirtualMachineImport
metadata:
  name: random-vm-test
  namespace: default
spec:
  virtualMachineName: "random-VM-test"
  networkMapping:
  - sourceNetwork: "shared"
    destinationNetwork: "default/vlan1"
  sourceCluster:
    name: devstack
    namespace: default
    kind: OpenstackSource
    apiVersion: migration.harvesterhci.io/v1beta1

• The import should be successful.

[mergify]

This pull request is now in conflict. Could you fix it @votdev? 🙏

[starbops]

LGTM, thanks!