allow the kube cert agent deployment's strategy type to be configured

[cfryanr]

This PR allows kube cert agent deployment's strategy type to be configured via the Concierge's static ConfigMap. The default is the old behavior, which is to leave it unspecified (which therefore uses the k8s default of "RollingUpdate"). If provided, we validate that the string provided in the ConfigMap is one of the two strings currently allowed in the k8s Deployment API.

The ConfigMap can be edited to use the new setting like this:

apiVersion: v1
kind: ConfigMap
metadata:
  name: pinniped-concierge-config
  namespace: concierge
data:
  pinniped.yaml: |
    # Other top-level settings trimmed for brevity.
    kubeCertAgent:
      # This is the new setting. It can be "Recreate", "RollingUpdate", or null.
      # When this setting is missing, then it is null.
      deploymentStrategyType: Recreate
      # Other kubeCertAgent settings trimmed for brevity.

Release note:

Yes, TBD.

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