[autoscaler] Fix bad reference when specifying IamInstanceProfile by name

[pdames]

Why are these changes needed?

Fixes a bug that otherwise results in a bad reference error when an IamInstanceProfile is specified by name for head or worker nodes in an AWS Autoscaler Config file, for example:

head_node:
  IamInstanceProfile:
    Name: MyRayClusterIamInstanceProfile

worker_nodes:
  IamInstanceProfile:
    Name: MyRayClusterIamInstanceProfile

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(