feat: truncate long labels in nodebalancers

[wbh1]

Nodebalancer backends can have a max label length of 32 chars.

If a k8s node's name is longer than 32 chars, creating a nodebalancer fails.

Example error:

E0307 22:31:47.275176       1 controller.go:307] error processing service traefik/traefik (will retry):
failed to ensure load balancer: [400] [configs[0].nodes[0].label] Length must be 3-32 characters;
[configs[0].nodes[1].label] Length must be 3-32 characters;
[configs[0].nodes[2].label] Length must be 3-32 characters;

General:

• Have you removed all sensitive information, including but not limited to access keys and passwords?
• Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines:

1. Does your submission pass tests?
2. Have you added tests?
3. Are you addressing a single feature in this PR?
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7. Have you explained your rationale for why this feature is needed?
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[dennisme]

+1 looks great!

[wbh1]

(Rebased after #141)