cli: restore 0.7.0-style whitespace printing.

@renepickhardt has a shell script we broke.  While we still produce
perfectly valid JSON, we should not gratuitously change tool output.

Plus, I prefer the missing space before the ':'.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_plugin.py

@@ -330,7 +330,7 @@ def test_utf8_passthrough(node_factory, executor):
                                    .format(l1.daemon.lightning_dir),
                                    'utf8', 'ナンセンス 1杯']).decode('utf-8')
     assert '\\u' not in out
-    assert out == '{\n   "utf8" : "ナンセンス 1杯"\n}\n'
+    assert out == '{\n   "utf8": "ナンセンス 1杯"\n}\n'
 
 
 def test_invoice_payment_hook(node_factory):