Error early when user is running with `---public-api-endpoint=true`

[simonswine]

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind feature

What happened:

• Having a cluster without an public api endpoint
• If you run tarmak --public-api-endpoint=true cluster kubectl get nodes it is in a loop trying to establish the connection

What you expected to happen:

• We should fail early and say that no public API endpoint has been enabled

How to reproduce it (as minimally and precisely as possible):

• Build cluster without public endpoint
• Run tarmak --public-api-endpoint=true cluster kubectl get nodes