Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Change rbd base image back to fedora 26

Due to the compatiblility issue for ceph server, it needs to change
back to fedora 26 from fedora 28, as a workaround.

**What this PR does / why we need it**:

**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 #67269

**Special notes for your reviewer**:
/sig storage
@dims @rootfs

**Release note**:
```release-note
NONE
```