Revert "bot_update: make --output_json required in script invocation."

This reverts commit 331a276b54.

Reason for revert: Looks like this is causing bots to fail on the waterfall

Example:
https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium%2FWin%2F60913%2F%2B%2Frecipes%2Fsteps%2Fbot_update%2F0%2Fstdout

Original change's description:
> bot_update: make --output_json required in script invocation.
> 
> It's already set by recipe, so this should have 0 difference in prod.
> 
> R=​iannucci@chromium.org
> 
> Bug:
> Change-Id: I1e1129b9748d6647e45eb26892c8e892019fe088
> Reviewed-on: https://chromium-review.googlesource.com/769164
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

TBR=iannucci@chromium.org,tandrii@chromium.org

Change-Id: I65d7665e45dde539f6436ffe8932319e03f96f25
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/777619
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -1004,7 +1004,7 @@ def parse_args():
                    help=('Deprecated.'))
   parse.add_option('--clobber', action='store_true',
                    help='Delete checkout first, always')
-  parse.add_option('--output_json', required=True,
+  parse.add_option('--output_json',
                    help='Output JSON information into a specified file')
   parse.add_option('--no_shallow', action='store_true',
                    help='Bypass disk detection and never shallow clone. '
@@ -1086,6 +1086,8 @@ def prepare(options, git_slns, active):
   step_text = '[%dGB/%dGB used (%d%%)]' % (used_disk_space_gb,
                                            total_disk_space_gb,
                                            percent_used)
+  if not options.output_json:
+    print '@@@STEP_TEXT@%s@@@' % step_text
   shallow = (total_disk_space < SHALLOW_CLONE_THRESHOLD
              and not options.no_shallow)
 
@@ -1147,6 +1149,7 @@ def checkout(options, git_slns, specs, revisions, step_text, shallow):
       ensure_no_checkout(dir_names, options.cleanup_dir)
       gclient_output = ensure_checkout(**checkout_parameters)
   except PatchFailed as e:
+    if options.output_json:
       # Tell recipes information such as root, got_revision, etc.
       emit_json(options.output_json,
                 did_run=True,
@@ -1179,6 +1182,7 @@ def checkout(options, git_slns, specs, revisions, step_text, shallow):
     got_revisions = { 'got_revision': 'BOT_UPDATE_NO_REV_FOUND' }
     #raise Exception('No got_revision(s) found in gclient output')
 
+  if options.output_json:
     # Tell recipes information such as root, got_revision, etc.
     emit_json(options.output_json,
               did_run=True,