Fail gracefully when MD5 is unavailable

The glanceclient currently assumes that MD5 will always be available.
This is not the case, however, in a FIPS-compliant environment.  This
patch enables the glanceclient to fail gracefully in such a case.

Closes-bug: #1871675
Change-Id: Ibd89989e06cc5be7da71f5f21561d73b5abc4104

glanceclient/common/utils.py

@@ -436,7 +436,14 @@ def integrity_iter(iter, checksum):
 
     :raises: IOError
     """
-    md5sum = hashlib.md5()
+    try:
+        md5sum = hashlib.new('md5')
+    except ValueError:
+        raise IOError(errno.EPIPE,
+                      'Corrupt image download. Expected checksum is %s '
+                      'but md5 algorithm is not available on the client' %
+                      checksum)
+
     for chunk in iter:
         yield chunk
         if isinstance(chunk, six.string_types):