export EXP_CAPG_GKE=true needs to be set for managed cluster to work, but is missing in the docs.

[mjnovice]

/kind bug

What steps did you take and what happened:
[A clear and concise description of what the bug is.]

What did you expect to happen:
we tried clusterctl init --infrastructure=gcp without the above env var doesn't work.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api version:
• Minikube/KIND version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):