REGTEST: set_ssl_cert.vtc: replace "echo" with "printf"

"echo -e" for some reason does not work on travis-ci, so let us switch
to "printf"

Fixes: #423

reg-tests/ssl/set_ssl_cert.vtc

@@ -42,7 +42,7 @@ shell {
 }
 
 shell {
-   echo -e "set ssl cert ${testdir}/common.pem <<\n$(cat ${testdir}/ecdsa.pem)\n" | socat "${tmpdir}/h1/stats" -
+   printf "set ssl cert ${testdir}/common.pem <<\n$(cat ${testdir}/ecdsa.pem)\n\n" | socat "${tmpdir}/h1/stats" -
    echo "commit ssl cert ${testdir}/common.pem" | socat "${tmpdir}/h1/stats" -
 }