fix: set defaults for AWS CP and Worker instanceType

[supershal]

Moved d2iq-labs#51
Addressed the pending review comments
What problem does this PR solve?:

• sets defaults for aws nodes CP: m5.xlarge and Workers: m5.2xlarge
• sets PLACEHOLDER value as defaults in AWSMachineTemplates because instanceType field is required by CAPA.
• sets instance type as required field in the schema since CAREN will be using the default if not provided by the user.
  update existing unit tests to use default schema
  NOTE
  Once this PR is finalized, I will create follow up PR to set instance profile and some other changes in the API types that would ensure that it creates variable schema from the object instead of creating new one.

Which issue(s) this PR fixes:
Fixes # #485

How Has This Been Tested?:

Tried to test locally using make e2e-test E2E_LABEL='provider:AWS && cni:Cilium' but local environment timed outs during initializations. Looking into fixing local environment (colima +docker +kind on mac)

Tested manually be creating AWS cluster.

aws-quick-start cluster class has following

 instanceType:
                      default: m5.xlarge
                      description: The AWS instance type to use for the cluster Machines
                      type: string
                  required:
                  - instanceType

and workers

                instanceType:
                  default: m5.2xlarge
                  description: The AWS instance type to use for the cluster Machines
                  type: string
              required:
              - instanceType

Created AWS cluster without adding instanceType variable, CAREN patched AWSMachineTemplate with default instanceType.

** Special notes for your reviewer:**

The approach taken in this PR will be used as guide to set default for other variables.