adds retries to fix len(dns). issue 79

resolve apel#79 . i was unsure which part to loop so i just repeated the whole chunk within the try statement.

ssm/agents.py

@@ -369,12 +369,24 @@ def get_dns(dn_file, log):
                 dns.append(line.strip())
             else:
                 log.warning('DN in incorrect format: %s', line)
+
+    # If no valid DNs, SSM cannot receive any messages.
+    if len(dns) == 0:
+        fails = 0
+        while fails < 3 and len(dns) == 0:
+            for line in lines:
+            if line.isspace() or line.strip().startswith('#'):
+                continue
+            elif line.strip().startswith('/'):
+                dns.append(line.strip())
+            else:
+                log.warning('DN in incorrect format: %s', line)
+        if len(dns) == 0:
+            raise Ssm2Exception('No valid DNs found in %s.  SSM will not start' % dn_file)
+
     finally:
         if f is not None:
             f.close()
-    # If no valid DNs, SSM cannot receive any messages.
-    if len(dns) == 0:
-        raise Ssm2Exception('No valid DNs found in %s.  SSM will not start' % dn_file)
 
     log.debug('%s DNs found.', len(dns))
     return dns