Touch attribute file to fix errors like:
  Can't open ./demoCA/index.txt.attr for reading,
  No such file or directory
  140553384993216:error:02001002:system library:
  fopen:No such file or directory:../crypto/bio/bss_file.c:72:
  fopen('./demoCA/index.txt.attr','r')
  140553384993216:error:2006D080:BIO routines:
  BIO_new_file:no such file:../crypto/bio/bss_file.c:79:
  Check that the request matches the signature