agent: Handle not found Software Installation

  In distribution systems, it is possible that master didn't indexed the Software Installation
  when we check, this change will consider we don't know the state of the Software Installation on
  this stage. The code will retry later anyway.

slapos/agent/tester.py

@@ -157,10 +157,15 @@ class SlapOSMasterCommunicator(object):
 
   @retryOnNetworkFailure
   def getSoftwareInstallationNews(self):
+    getter_link = None
     for si in self.getSoftwareInstallationList():
       if si["title"] == self.url:
         getter_link = si["href"]
         break
+
+    # We could not find the document, so it is probably too soon.
+    if getter_link is None:
+      return ""
     
     result = self.hateoas_navigator.GET(getter_link)
     action_object_slap_list = json.loads(result)['_links']['action_object_slap']