Use correct filename for index & serial file when setting permissions

There was a cut-n-paste bug where self._set_permissions() was called
with the exact same filename, self.ssl_config_file_name, instead of
the index and serial filenames. This patch uses the index and serial
filenames as was the original intent.

Change-Id: I571c766ac746bbbc1bedfdf1ff2b1b86363a0af0
Fixes: bug #1206254
Merge "Use correct filename for index & serial file when setting permissions"