Improve debug logs. (#3126)

Print request and response bodies with newlines, rather than all on one line.
Remove "Omitted empty field" log, which gets logged meaninglessly for every JSON
serialization.
Remove duplicated logging of responses.
Log the base64 version of the nonce, rather than turning it into bytes and
logging the backslash-escaped version of those bytes.
Only pass -vv in tests.

tests/integration/_common.sh

@@ -32,6 +32,6 @@ certbot_test_no_force_renew () {
         --agree-tos \
         --register-unsafely-without-email \
         --debug \
-        -vvvvvvv \
+        -vv \
         "$@"
 }