Automatic merge from submit-queue
Use ubuntu-slim to reduce size of the iperf:e2e image
from
```
gcr.io/google_containers/iperf e2e 8b3cc7064090 5 weeks ago 737.9 MB
```
to
```
gcr.io/google_containers/iperf e2e 204325491636 33 seconds ago 61.09 MB
```
related to https://github.com/kubernetes/kubernetes/pull/25784#issuecomment-221706886
ping @bprashanth