metallb addon: Ask user for config values even if already set

[spowelljr]

Closes #12051

Problem:
Currently, once you configure the metallb addon you can't change the values unless you manully edit the config.json file or start a new cluster.

Solution:
Now asks user to enter config values every time minikube addons configure metallb is run.

Before (doesn't ask for values second time):

$ minikube addons configure metallb
-- Enter Load Balancer Start IP: 1.1.1.1
-- Enter Load Balancer End IP: 1.1.1.1
    ▪ Using image metallb/speaker:v0.9.6
    ▪ Using image metallb/controller:v0.9.6
✅  metallb was successfully configured

$ minikube addons configure metallb
    ▪ Using image metallb/speaker:v0.9.6
    ▪ Using image metallb/controller:v0.9.6
✅  metallb was successfully configured

After (asks for values second time):

$ minikube addons configure metallb
-- Enter Load Balancer Start IP: 1.1.1.1
-- Enter Load Balancer End IP: 1.1.1.1
    ▪ Using image metallb/speaker:v0.9.6
    ▪ Using image metallb/controller:v0.9.6
✅  metallb was successfully configured

$ minikube addons configure metallb
-- Enter Load Balancer Start IP: 1.1.1.1
-- Enter Load Balancer End IP: 1.1.1.1
    ▪ Using image metallb/speaker:v0.9.6
    ▪ Using image metallb/controller:v0.9.6
✅  metallb was successfully configured

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This pull-request has been approved by: medyagh, spowelljr

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