Fix pv patch continuously problem (#2769)

[ti-srebot]

cherry-pick #2769 to release-1.1

---

What problem does this PR solve?

Currently, pv would be continuously patched which is not we want. I found that there are mainly 2 problems in the current logic:

1. the selector for the tidbcluster pvc would select all the pvc including tidbmonitor' pvc.

case v1alpha1.TiDBClusterKind:	
     l, err = label.New().Instance(instanceName).Selector()	
     if err != nil {	
         return err	
     }

2. If we didn't define the PersistentVolumeReclaimPolicy in the Spec, the comparing here will be not equal forever.

if pv.Spec.PersistentVolumeReclaimPolicy == policy {
    continue
}

What is changed and how does it work?

Make PersistentVolumeReclaimPolicy as pointer and fix the selector logic.

Does this PR introduce a user-facing change?:

Fix the bug that the `PV` managed by Operator would be patched continuously

[Yisaer]

/merge

[ti-srebot]

/run-all-tests