Add note on using printable chars for swift_hash_path_suffix/prefix

Using unprintable characters for swift_hash_path_prefix/suffix might lead to
hard problems when parsing of these values changes, for example due to newer
Python versions or changes in the parsers itself. Let's avoid this and add a
note that deployers should use printable strings for these values.

Change-Id: I976982b753b6af831ab91d7190f50f8f15bf73bf

doc/saio/swift/swift.conf

@@ -1,5 +1,6 @@
 [swift-hash]
 # random unique strings that can never change (DO NOT LOSE)
+# Use only printable chars (python -c "import string; print(string.printable)")
 swift_hash_path_prefix = changeme
 swift_hash_path_suffix = changeme
 

doc/source/overview_policies.rst

@@ -286,6 +286,7 @@ example configuration.::
 
         [swift-hash]
         # random unique strings that can never change (DO NOT LOSE)
+        # Use only printable chars (python -c "import string; print(string.printable)")
         swift_hash_path_prefix = changeme
         swift_hash_path_suffix = changeme
 

etc/swift.conf-sample

@@ -4,6 +4,7 @@
 # the hashing algorithm when determining data placement in the cluster.
 # These values should remain secret and MUST NOT change
 # once a cluster has been deployed.
+# Use only printable chars (python -c "import string; print(string.printable)")
 
 swift_hash_path_suffix = changeme
 swift_hash_path_prefix = changeme