mock: use 'private' mount option, not 'rprivate' (el7 fix)

The 'mount --rbind -o rprivate' on EL7 does not work as we expected [1];
it looks like it actually reverses the '--rbind' option and does only
'--bind' (filled rhbz#1815534).  Using just 'private' seems to do the
same (good) thing on both EL7 and modern Fedora.

This fixes local baseurl=file:// repositories with bootstrap, and
yum_chache plugin on EL7.

Fixes: #381, #536
Relates: https://bugzilla.redhat.com/show_bug.cgi?id=1815534

mock/py/mock.py

@@ -747,15 +747,15 @@ def main():
         # add the extra bind mount to the outer chroot
         inner_mount = bootstrap_buildroot.make_chroot_path(buildroot.make_chroot_path())
 
-        # Hide re-mounted chroot from host by rprivate tmpfs.
+        # Hide re-mounted chroot from host by private tmpfs.
         buildroot.mounts.managed_mounts.append(
             FileSystemMountPoint(filetype='tmpfs',
                                  device='hide_root_in_bootstrap',
                                  path=inner_mount,
-                                 options="rprivate"))
+                                 options="private"))
         buildroot.mounts.managed_mounts.append(
             BindMountPoint(buildroot.make_chroot_path(), inner_mount,
-                           recursive=True, options="rprivate"))
+                           recursive=True, options="private"))
 
     signal.signal(signal.SIGTERM, partial(handle_signals, buildroot))
     signal.signal(signal.SIGPIPE, partial(handle_signals, buildroot))