Support bootstrap only additional listener

[hamzy]

Add an option to the AdditionalListenerSpec to signify that this pool should only contain the boostrap node. An example of this would be for port 22 since every master node also has ssh but we need the public load balancer to only connect to the bootstrap ssh server and not randomly to any one of the four nodes.

What this PR does / why we need it:

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/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

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[k8s-ci-robot]

Hi @hamzy. Thanks for your PR.

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[Karthik-K-N]

/ok-to-test

[Karthik-K-N]

Overall I like the idea just thinking, Can we generalise this rather than sticking only to bootstrap configuration?
Say tomorrow we want to configure something only on the master nodes then we need to make the changes again.
I would like to know the opinions
@dharaneeshvrd @Amulyam24 @Prajyot-Parab