Add DefaultBindParameters to plans

[carolynvs]

Similar to how we added DefaultProvisionParameters to plans in #2282, we also want the same type of functionality but for the parameters on ServiceBinding resources.

See the proposal for additional context and details: https://github.com/carolynvs/service-catalog/blob/default-service-plan-proposal/docs/proposals/default-service-plans.md#binding-with-defaults. The scope of this issue is only the changes to the resource definitions and the controller, not svcat.

An operator can edit a plan and define defaultBindParameters:

apiVersion: servicecatalog.k8s.io/v1beta1
kind: ClusterServicePlan
metadata:
  name: "427559f1-bf2a-45d3-8844-32374a3e58aa"
spec:
  clusterServiceClassRef:
    name: "997b8372-8dac-40ac-ae65-758b4a5075a5"
  externalName: basic50
  # spec truncated for clarity
  defaultBindParameters:
    user: reader
    hasExec: false 

Then when someone creates a binding for that plan, the parameters from the plan are merged with the parameters (if any) on the binding:

apiVersion: servicecatalog.k8s.io/v1beta1
kind: ServiceBinding
metadata:
  name: mydb-admin
spec:
  instanceRef:
    name: mydb
  parameters:
    user: admin

The final ServiceBinding that is used to for the bind operation with the broker:

apiVersion: servicecatalog.k8s.io/v1beta1
kind: ServiceBinding
metadata:
  name: mydb-admin
spec:
  instanceRef:
    name: mydb
  parameters:
    user: admin
    hasExec: false

[carlory]

#dibs

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[fejta-bot]

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/lifecycle stale

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[mszostok]

/remove-lifecycle rotten
/lifecycle frozen

[mrbobbytables]

This project is being archived, closing open issues and PRs.
Please see this PR for more information: kubernetes/community#6632