[bugfix] - handle LoadBalancer deletion if service type changes to no longer be a LoadBalancer

[AshleyDumaine]

There's currently a bug that leaves an orphaned NodeBalancer if a Service's type changes to no longer be LoadBalancer. If the type is changed back, a new NodeBalancer is created.
This PR detects a change in Service type and deletes the NodeBalancer if necessary.

e.g. when doing a kubectl edit on a Service to instead be a NodePort type:

I0521 14:43:02.423709       1 service_controller.go:61] ServiceController will handle service (test-ns2/test-svc) LoadBalancer deletion
I0521 14:43:02.423736       1 service_controller.go:111] ServiceController handling service (test-ns2/test-svc) deletion
I0521 14:43:02.424889       1 controller.go:969] Removing finalizer from service test-ns2/test-svc
I0521 14:43:02.425893       1 event.go:376] "Event occurred" object="test-ns2/test-svc" fieldPath="" kind="Service" apiVersion="v1" type="Normal" reason="Type" message="LoadBalancer -> NodePort"
I0521 14:43:02.431744       1 event.go:376] "Event occurred" object="test-ns2/test-svc" fieldPath="" kind="Service" apiVersion="v1" type="Normal" reason="DeletedLoadBalancer" message="Deleted load balancer"
I0521 14:43:02.527551       1 loadbalancers.go:498] found NodeBalancer (675801) for service (test-ns2/test-svc) via IPv4 (172.232.16.7)
I0521 14:43:02.668512       1 loadbalancers.go:471] successfully deleted NodeBalancer (675801) for service (test-ns2/test-svc)

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[rahulait]

LGTM, works fine and lb ip gets deleted if service type is changed.