fix:check pool name before add

[jiuker]

check pool name before add
related #1626

[cniackz]

Hello @jiuker

How can I test this PR?...

I tested locally with these changes, but when I add duplicated pool by editing the tenant spec, I can add it and I don't see "pool exists" error:

I0609 20:35:06.998776       1 controller.go:70] Starting MinIO Operator
I0609 20:35:07.005589       1 main-controller.go:272] Setting up event handlers
I0609 20:35:07.011889       1 main-controller.go:481] Using Kubernetes CSR Version: v1
I0609 20:35:07.011913       1 main-controller.go:501] STS Api server is not enabled, not starting
I0609 20:35:07.011966       1 leaderelection.go:248] attempting to acquire leader lease minio-operator/minio-operator-lock...
I0609 20:35:07.014675       1 leaderelection.go:258] successfully acquired lease minio-operator/minio-operator-lock
I0609 20:35:07.014701       1 main-controller.go:530] minio-operator-8c68f99bc-vt885: I am the leader, applying leader labels on myself
I0609 20:35:07.014728       1 main-controller.go:385] Waiting for Upgrade Server to start
I0609 20:35:07.014735       1 main-controller.go:389] Starting Tenant controller
I0609 20:35:07.014737       1 main-controller.go:392] Waiting for informer caches to sync
I0609 20:35:07.014744       1 main-controller.go:397] Starting workers
I0609 20:35:07.014751       1 main-controller.go:425] Console TLS is not enabled
I0609 20:35:07.014761       1 main-controller.go:347] Starting HTTP Upgrade Tenant Image server
I0609 20:35:07.029889       1 operator.go:115] Checking if this is OpenShift Environment to append the certificates...
I0609 20:35:07.029905       1 operator.go:119] csr-signer secret wasn't found, very likely this is not OpenShift but k8s Vanilla or other...
I0609 20:35:07.030104       1 operator.go:115] Checking if this is OpenShift Environment to append the certificates...
I0609 20:35:07.030137       1 operator.go:119] csr-signer secret wasn't found, very likely this is not OpenShift but k8s Vanilla or other...
I0609 20:35:07.411874       1 status.go:89] Hit conflict issue, getting latest version of tenant
E0609 20:35:12.053127       1 main-controller.go:666] error syncing 'default/cesar': Operation cannot be fulfilled on statefulsets.apps "cesar-pool-1": the object has been modified; please apply your changes to the latest version and try again

[cniackz]

I will approve this change based on the code review; but I am having troubles to understand how this can be tested. I will talk to Jiuker directly to learn from him the testing steps and document.