Adding ascii_only and dd sparse file

Creating sparse files for loopback images can be done with the dd
command. The ascii_only strips all non-ascii characters out of a
string.

dd_create_sparse_file

+# Create sparse.img with 100G (1K * 104857600) and
+# actually transfer 0 K into it (count).
+dd of=sparse.img bs=1k seek=104857600 count=0

python_utils/ascii_only.py

+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+import re
+
+logger = logging.getLogger(__name__)
+
+ascii_characters = [chr(i) for i in range(32,127)]
+ascii_char_rgx = re.compile("{}".format(re.escape(ascii_characters)))
+
+
+def ascii_only(incoming_string):
+    return control_char_rgx.sub("", incoming_string)