Create pluggable mechanism for service type LoadBalancer in CPI

[frapposelli]

/kind feature

Create a pluggable mechanism for service type load balancer in CPI to support multiple loadbalancers on vSphere.

ref: #272

[fejta-bot]

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/lifecycle stale

[frapposelli]

/remove-lifecycle stale
/lifecycle frozen

[YanzhaoLi]

Can we close this issue based on this PR: #292