testsys: refactor and allow overrides in shared 'migration_crd' generation

[etungsten]

Issue number:
N/A

Description of changes:

    testsys: refactor and allow aws region override in 'migration_crd'
    
    Migration testing for VMware variants require a region for SSM to
    dispatch upgrade/downgrade RunCommands to managed instances in vSphere.
    
    Unlike ECS or EKS, TestSys cannot assume the cluster resource CRD
    includes a region for vmware-k8s testing. Therefore we pass 'us-west-2'
    as the override region when creating the migration test CRD.

    testsys: add param to 'migration_crd' to specify instance id field name
    
    The TestSys EC2 resource CRD stores the instance IDs of created
    instances in a field named "ids", but the vSphere VM resource CRD stores
    the instance IDs of the managed instances in a field named
    "instance_ids".

Before these changes, migration testing on vSphere will fail because the templated fields for migration test don't exist in the VM resource CRD:

[2022-12-21T21:17:05Z ERROR migration_test_agent] client error: Unable to resolve config templates: An error occured while resolving the config: No field 'region' in created resource

Testing done:

The migration tests successfully kick-off and finish:

Every 2.0s: testsys --kubeconfig /home/etung/1.12.0-release/testsys_kubeconfig.yaml status                                                                                   Thu Dec 22 00:09:47 2022

 NAME                                                      TYPE                         STATE                         PASSED                     SKIPPED                     FAILED
 x86-64-vmware-k8s-124                                     Resource                     completed
 x86-64-vmware-k8s-124-1-initial                           Test                         pass                          1                          6972                        0
 x86-64-vmware-k8s-124-2-migrate                           Test                         pass                          2                          0                           0
 x86-64-vmware-k8s-124-3-migrated                          Test                         pass                          1                          6972                        0
 x86-64-vmware-k8s-124-4-migrate                           Test                         pass                          2                          0                           0
 x86-64-vmware-k8s-124-5-final                             Test                         pass                          1                          6972                        0
 x86-64-vmware-k8s-124-test                                Test                         running                       2                          0                           0
 x86-64-vmware-k8s-124-vms-conformance                     Resource                     completed

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[ecpullen]

I'd prefer to keep this in aws_resources.rs to signify that this migration is performed using aws tools.

[etungsten]

I thought about this a little bit more and I think it makes sense to separate out the migration CRD construction from aws_resources since the migration CRD is for a test CRD and the test itself is not limited to just run on EKS/ECS. Sure it may use AWS tools but we also use AWS tools in VMware VMs and cluster resource as well.