More sensible defaults for webhook environment variables

[kispaljr]

Remove hardwired string literals as the default values of WEBHOOK_SERVICE_NAME and WEBHOOK_SERVICE_NAMESPACE environment variables.

This PR introduces the following changes in behavior:

The value of WEBHOOK_SERVICE_NAME is determined by one of the followings (in precedence order):

• the WEBHOOK_SERVICE_NAME environment variable
• the name of the service specified in the APIService object named v1alpha1.porch.kpt.dev

The value of WEBHOOK_SERVICE_NAMESPACE is determined by one of the followings (in precedence order):

• the WEBHOOK_SERVICE_NAMESPACE environment variable
• the CERT_NAMESPACE environment variable (deprecated, please don't use this!!!)
• the namespace of the service specified in the APIService object named v1alpha1.porch.kpt.dev
• namespace of the current process (if running in a pod)

[kispaljr]

/retest

[kispaljr]

/retest

[kispaljr]

/retest

[kispaljr]

/retest

[kispaljr]

/retest

[efiacor]

/approve
/lgtm

[nephio-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: efiacor, kispaljr

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