[ECS] Fix issue with multiple networks

[lego963]

Summary of the Pull Request

Fix issue with reading networks in resource/opentelekomcloud_compute_instance_v2
Fixes: #1039

PR Checklist

• Refers to: [ECS][IMPORT] opentelekomcloud_compute_instance_v2 seams to not work with multiple Subnet in VPC #1039
• Tests added/passed.
• Documentation updated.
• Schema updated.

Acceptance Steps Performed

=== RUN   TestAccComputeV2Instance_basic
--- PASS: TestAccComputeV2Instance_basic (140.33s)
=== RUN   TestAccComputeV2Instance_multiSecgroup
--- PASS: TestAccComputeV2Instance_multiSecgroup (149.64s)
=== RUN   TestAccComputeV2Instance_bootFromImage
--- PASS: TestAccComputeV2Instance_bootFromImage (79.78s)
=== RUN   TestAccComputeV2Instance_bootFromVolume
--- PASS: TestAccComputeV2Instance_bootFromVolume (78.77s)
=== RUN   TestAccComputeV2Instance_changeFixedIP
--- PASS: TestAccComputeV2Instance_changeFixedIP (80.90s)
=== RUN   TestAccComputeV2Instance_bootFromVolumeVolume
--- PASS: TestAccComputeV2Instance_bootFromVolumeVolume (100.41s)
=== RUN   TestAccComputeV2Instance_stopBeforeDestroy
--- PASS: TestAccComputeV2Instance_stopBeforeDestroy (126.18s)
=== RUN   TestAccComputeV2Instance_metadata
--- PASS: TestAccComputeV2Instance_metadata (137.88s)
=== RUN   TestAccComputeV2Instance_timeout
--- PASS: TestAccComputeV2Instance_timeout (87.94s)
=== RUN   TestAccComputeV2Instance_autoRecovery
--- PASS: TestAccComputeV2Instance_autoRecovery (127.58s)
=== RUN   TestAccComputeV2Instance_crazyNICs
--- PASS: TestAccComputeV2Instance_crazyNICs (231.48s)
=== RUN   TestAccComputeV2Instance_initialStateActive
--- PASS: TestAccComputeV2Instance_initialStateActive (215.49s)
=== RUN   TestAccComputeV2Instance_initialStateShutoff
--- PASS: TestAccComputeV2Instance_initialStateShutoff (287.69s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.

• golang-make-test : SUCCESS in 8m 44s
• golang-make-vet : SUCCESS in 8m 16s
• goreleaser-build : SUCCESS in 7m 54s