Take the list of arches for -build Koji tag from buildrequired modules.

Currently, we are using just `conf.arches` and `conf.base_module_arches`
to define the list of arches for which the RPMs in a submitted module are
built. This is not enough, because RCM needs to generate modules based
on the base modules which should use different arches.

This commit changes the MBS to take the list of arches from the buildrequired
module build. It checks the buildrequires for "privileged" module or base
module and if it finds such module, it queries the Koji to find out the list
of arches to set for the module.

The "privileged" module is a module which can override base module arches
or disttag. Previously, these modules have been defined by
`allowed_disttag_marking_module_names` config option. In this commit,
this has been renamed to `allowed_privileged_module_names`.

The list of arches are stored per module build in new table represented
by ModuleArch class and are m:n mapped to ModuleBuild.

module_build_service/builder/MockModuleBuilder.py

@@ -52,6 +52,20 @@ from module_build_service import models
 logging.basicConfig(level=logging.DEBUG)
 
 
+def detect_arch():
+    """
+    Helper method to detect the local host architecture. Fallbacks to `conf.arch_fallback`.
+    """
+    if conf.arch_autodetect:
+        arch_detected = platform.machine()
+        if arch_detected:
+            return arch_detected
+
+        log.warning("Couldn't determine machine arch. Falling back to configured arch.")
+
+    return conf.arch_fallback
+
+
 class MockModuleBuilder(GenericBuilder):
     backend = "mock"
     # Global build_id/task_id we increment when new build is executed.
@@ -94,15 +108,7 @@ class MockModuleBuilder(GenericBuilder):
         self.koji_session = None
 
         # Auto-detect arch (if possible) or fallback to the configured one
-        if conf.arch_autodetect:
-            arch_detected = platform.machine()
-            if arch_detected:
-                self.arch = arch_detected
-            else:
-                log.warning("Couldn't determine machine arch. Falling back to configured arch.")
-                self.arch = conf.arch_fallback
-        else:
-            self.arch = conf.arch_fallback
+        self.arch = detect_arch()
         log.info("Machine arch setting: {}".format(self.arch))
 
         # Create main directory for this tag
@@ -149,6 +155,17 @@ class MockModuleBuilder(GenericBuilder):
         # Workaround koji specific code in modules.py
         return {"name": self.tag_name}
 
+    @classmethod
+    def get_module_build_arches(cls, module):
+        """
+        :param ModuleBuild module: Get the list of architectures associated with
+            the module build in the build system.
+        :return: list of architectures
+        """
+        # Return local architecture, because all the modules built locally are built
+        # just against this architecture.
+        return [detect_arch()]
+
     def _createrepo(self, include_module_yaml=False):
         """
         Creates the repository using "createrepo_c" command in the resultsdir.