Remove old webhookconfiguration if it alreasy exist before create new one

[vincent-pli]

What this PR does / why we need it:
In #145 , we use self-signed certificate for ValidatingWebhookConfiguration, that's means we will generate self-signed CA when vc-manager startup.
Based on current implements, when creating ValidatingWebhookConfiguration and found it already exist we will give up to create a new one and expect to use the older one, that's will cause problem since the private key is re-generated by new CA but in old ValidatingWebhookConfiguration included the old CA, that's will case the webhook raise:

root@rentz1:~# kubectl delete VirtualCluster vc-sample-1
Error from server (InternalError): Internal error occurred: failed calling webhook "virtualcluster.validating.webhook": Post "https://virtualcluster-webhook-service.vc-manager.svc:9443/validate-tenancy-x-k8s-io-v1alpha1-virtualcluster?timeout=30s": x509: certificate signed by unknown authority (possibly because of "crypto/rsa: verification error" while trying to verify candidate authority certificate "tenancy.x-k8s.io")

In my env, the vc-manager has restarted by unknown reason, then I hit the error mentioned upper.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[k8s-ci-robot]

Hi @vincent-pli. Thanks for your PR.

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[charleszheng44]

Good catch!
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: charleszheng44, vincent-pli

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[charleszheng44]

/ok-to-test