plugins: ability to skip the tagging

Add a new command line option `--skip-tag` that will skip tagging
after a successful build. The help text is copied from the same
option of other sub-commands in the koji client. The hub plugin's
jsonschema was updated accordingly, and the builder plugin will
skip the tag if the option was requested.
Tests were added or augmented for all three plugins to test the
new option.

plugins/builder/osbuild.py

@@ -416,7 +416,9 @@ class OSBuildImage(BaseTaskHandler):
         if not status.is_success:
             raise koji.BuildError(f"Compose failed (id: {cid})")
 
-        self.tag_build(target_info["dest_tag"], bid)
+        # Build was successful, tag it
+        if not opts.get('skip_tag'):
+            self.tag_build(target_info["dest_tag"], bid)
 
         result = {
             "composer": {