Add new publicSubnetIds and privateSubnetIds cluster options. Also, update tests to use new awsx.ec2.Vpc API and new subnet options

[metral]

Proposed changes

1. feat(subnetIds): add new opts: publicSubnetIds and privateSubnetIds
   This change intends to tackle 2 cases:

   • Simplifying cluster subnet defintion by explicitly accepting both
     public and private subnets, and
   • To properly build the cluster dependency graph w.r.t. the subnets used by the
     cluster. This should produce expected results for nodegroup subnet definition
     between initial updates and proceeding empty preview steps.

   Cluster worker subnet definition benefits from having individual publicSubnetIds
   and privateSubnetIds options, as they are:

   • Explicit in defintion of which subnets are being used for a given subnet class.
   • Inline with user practice, and
   • computeWorkerSubnets() in the NodeGroup class is an approximation
     of the which subnets belong to which class, and it can occassionaly experience
     non-deterministic issues.

2. fix(tests): update awsx.Network -> awsx.ec2.Vpc & use specific subnetIds

   • Update tests that use VPCs created with awsx.Network to the newer
     awsx.ec2.Vpc API.
   • Remove any awsx.ec2.Vpc settings already set by defaults.
   • Update EKS cluster definitions that used the eks.Cluster.subnetIds to
     now use the new subnet options: eks.Cluster.publicSubnetIds and eks.Cluster.privateSubnetIds.

3. test(awsx-network-and-subnetIds): add test of older VPC & network setup
   Add new test which creates EKS clusters using the previous awsx.Network API
   and related cluster network setups.

   This test creates two types of clusters that use the previous network API setup:

   • One cluster uses a VPC from the awsx.Network API, and cluster subnetIds.
   • One cluster uses a VPC from the awsx.Network API, cluster
     subnetIds, and a node group with the nodeSubnetIds override option.

Related issues

Closes #199
Closes #179