Allow configuring the number of parallel submissions to Koji

When NUM_CONCURRENT_BUILDS is 0, then multi-threading is disabled
when submitting builds to Koji. This is not acceptable, so this
commit makes the number of threads configurable.

module_build_service/config.py

@@ -653,7 +653,13 @@ class Config(object):
                     "x, y, and z (represents the version). The third value is an optional template "
                     "string that represent the Product Pages release for major releases "
                     "(e.g. 8.0.0). After the first match, the rest will be ignored."
-        }
+        },
+        "num_threads_for_build_submissions": {
+            "type": int,
+            "default": 5,
+            "desc": "The number of threads when submitting component builds to an external build "
+                    "system.",
+        },
     }
 
     def __init__(self, conf_section_obj):
@@ -907,3 +913,10 @@ class Config(object):
                     )
 
         self._product_pages_module_streams = d
+
+    def _setifok_num_threads_for_build_submissions(self, i):
+        if not isinstance(i, int):
+            raise TypeError("NUM_THREADS_FOR_BUILD_SUBMISSIONS needs to be an int")
+        if i < 1:
+            raise ValueError("NUM_THREADS_FOR_BUILD_SUBMISSIONS must be >= 1")
+        self._num_threads_for_build_submissions = i

module_build_service/utils/batches.py

@@ -150,9 +150,7 @@ def continue_batch_build(config, module, session, builder, components=None):
         components_to_build.append(c)
 
     # Start build of components in this batch.
-    max_workers = 1
-    if config.num_concurrent_builds > 0:
-        max_workers = config.num_concurrent_builds
+    max_workers = config.num_threads_for_build_submissions
     with concurrent.futures.ThreadPoolExecutor(max_workers=max_workers) as executor:
         futures = {
             executor.submit(start_build_component, builder, c): c for c in components_to_build