fix: always set AMI ID for custom LaunchTemplates with ManagedNodeGroups

[rquitales]

Proposed changes

1163 enabled setting custom user data in MIME format. However, AWS will attempt to merge the user data with its own copy if an AMI ID is not supplied in the LaunchTemplate. This results in the nodes being unable to join the cluster due to conflicting bootstrap script runs. This PR explicitly sets the ami id within the launch template.

Testing:

• Updated the existing tests to provision a cluster within a VPC to surface any node joining issues which CI was previously ignoring

Fixes: #1165

[github-actions[bot]]

Does the PR have any schema changes?

Looking good! No breaking changes found. No new resources/functions.