[AWS] Adding filter for default aws vpc if not specified

[clayrosenthal]

Adding a filter for the default AWS VPC when no VPC is specified. This is matching the docs:

aws:
  # ...
  # VPC to use in each region (optional).
  #
  # If this is set, SkyPilot will only provision in regions that contain a VPC
  # with this name (provisioner automatically looks for such regions).
  # Regions without a VPC with this name will not be used to launch nodes.
  #
  # Default: null (use the default VPC in each region).
  vpc_name: skypilot-vpc

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[romilbhardwaj]

Thanks @clayrosenthal! Pushed some commits to update error messages and docs.

Noting that with this change, SkyPilot will only use either the specified VPC (config.aws.vpc_name), or the default VPC. Other VPCs will not be used unless explicitly specified.