Create one AWS VPC/IGW/PlacementGroup per region.
Create one AWS VPC/IGW/PlacementGroup per region.

VPCs, Internet Gateways, and Placement groups are regional resources.
This change creates a single VPC/IGW/Placement Group per region (via
AwsRegionalNetwork), then creates a per-AZ subnet.

Tested with:

    python pkb.py --benchmarks netperf --cloud AWS \
      --zones us-west-1a,us-west-1c \
      --ip_addresses INTERNAL \
      --netperf_test_length 10

    python pkb.py --benchmarks netperf --cloud AWS \
      --zones us-west-1 \
      --ip_addresses INTERNAL \
      --netperf_test_length 10

AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,AdamIsrael/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,meteorfox/PerfKitBenchmarker,mateusz-blaszkowski/PerfKitBenchmarker