proxy test from ubuntu_k8s_unit_tests failed on F-gcp-5.4

Issue found on 5.4.0-1016.16-gcp

Test failed with:
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:45760: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:55578: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:39756: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:39630: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:43580: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:52204: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:44876: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:35424: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:34632: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:55718: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 --- FAIL: TestDialURL (0.05s)
 dial_test.go:139: insecure: expected no error, got "remote error: tls: handshake failure"
 dial_test.go:139: insecure, custom dial: expected no error, got "remote error: tls: handshake failure"
 dial_test.go:139: secure with roots, custom dial: expected no error, got "remote error: tls: handshake failure"
 dial_test.go:139: secure with roots: expected no error, got "remote error: tls: handshake failure"
 dial_test.go:139: secure with matched server: expected no error, got "remote error: tls: handshake failure"
 dial_test.go:139: secure with matched server, custom dial: expected no error, got "remote error: tls: handshake failure"
 2020/06/13 18:01:34 http: TLS handshake error from 127.0.0.1:59702: tls: failed to sign handshake: crypto/rsa: key size too small for PSS signature
 --- FAIL: TestProxyUpgrade (0.00s)
 upgradeaware_test.go:492: error dialing backend: remote error: tls: handshake failure
 upgradeaware_test.go:466: https (invalid hostname + InsecureSkipVerify): websocket dial err: websocket.Dial ws://127.0.0.1:43175/some/path: bad status
 FAIL
 FAIL k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/util/proxy 0.077s