Fix pull credentials bug, fix job details parse bug and add some better logging

buildman/jobutil/buildjob.py

@@ -33,6 +33,11 @@ class BuildJob(object):
   def repo_build(self):
     return self._load_repo_build()
 
+  @property
+  def pull_credentials(self):
+    """ Returns the pull credentials for this job, or None if none. """
+    return self.job_details.get('pull_credentials')
+
   @property
   def build_config(self):
     try: