Replace parse_strtime with parse_isotime in older/newer

There's no reason to limit parsing to strtime format as isotime format
is better and than parsing isotime also parses strtime formatted
strings.

Change-Id: I94dc4e9fa08b10139e3ef27fa94e33e0aa124110

oslo_utils/timeutils.py

@@ -119,7 +119,7 @@ def normalize_time(timestamp):
 def is_older_than(before, seconds):
     """Return True if before is older than seconds."""
     if isinstance(before, six.string_types):
-        before = parse_strtime(before).replace(tzinfo=None)
+        before = parse_isotime(before).replace(tzinfo=None)
     else:
         before = before.replace(tzinfo=None)
 
@@ -129,7 +129,7 @@ def is_older_than(before, seconds):
 def is_newer_than(after, seconds):
     """Return True if after is newer than seconds."""
     if isinstance(after, six.string_types):
-        after = parse_strtime(after).replace(tzinfo=None)
+        after = parse_isotime(after).replace(tzinfo=None)
     else:
         after = after.replace(tzinfo=None)