Add a way to auto import cluster into rancher

[eva-vashkevich]

Currently if we try to add "cluster-api.cattle.io/rancher-auto-import": "true" label, UI does not allow us to do it because it is counted as system label. We should provide a way to do it

[valaparthvi]

The new Auto-import cluster to Rancher checkbox in Create Cluster wizard auto-imports the cluster. Is that not what this issue is about? I tested on 0.8.3-rc2 and auto-importing works as expected.