rancher: Kubernetes version dropdown is empty

Version:

2.3.2

Steps:

1. Start rancher server
2. Create a RKE cluster

Results:

Kubernetes version dropdown is empty

Expected:

Users should be able to see k8s versions.

More Info:

If the connection between rancher server and github is bad, rancher server might take 30 mins to finish the git clone k8s-metadata. During this 30 mins, users will not be able to see any k8s version in create cluster page.