make /etc/yum.conf symlink to /etc/yum/yum.conf

Because Amazon Linux prefers /etc/yum.conf which is just touched and empty, while RHEL prefers /etc/yum/yum.conf
and ignore the touched /etc/yum.conf when the former exists.

This resolve issue #82

py/mockbuild/buildroot.py

@@ -509,10 +509,13 @@ def _setup_devices(self):
     def _setup_files(self):
         # self.root_log.debug('touch required files')
         for item in [self.make_chroot_path('etc', 'fstab'),
-                     self.make_chroot_path('etc', 'yum.conf'),
+                     self.make_chroot_path('etc', 'yum', 'yum.conf'),
                      self.make_chroot_path('etc', 'dnf.conf'),
                      self.make_chroot_path('var', 'log', 'yum.log')]:
             util.touch(item)
+        short_yum_confpath = self.make_chroot_path('etc', 'yum.conf')
+        if not os.path.exists(short_yum_confpath):
+            os.symlink("yum/yum.conf", short_yum_confpath)
 
     @traceLog()
     def _setup_files_postinstall(self):