Add config option for algo to use in temp_url tests

This is so to configure which hashlib algorithm should be used for
the temp_url tests, defaulting to sha256 but preserving the
ability to use sha1 for compatibility with other Swift implementations.

Change-Id: Ia4923d47870fcb914a33adecb7155763ec1d0b2f
diff --git a/tempest/config.py b/tempest/config.py
index 4098f32..f986ddb 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -1164,6 +1164,11 @@
     cfg.BoolOpt('discoverability',
                 default=True,
                 help="Execute discoverability tests"),
+    cfg.StrOpt('tempurl_digest_hashlib',
+               default='sha256',
+               help="Hashing algorithm to use for the temp_url tests. "
+                    "Needs to be supported both by Swift and the "
+                    "hashlib module, for example sha1 or sha256"),
 ]