set truthy values for Requests and Certificates

certificate.py

@@ -17,6 +17,9 @@ class Certificate(object):
     def __repr__(self):
         return ( "%s %s" % ( str(self.__class__.__name__), str(self.cert_id) ) )
 
+    def __bool__(self):
+        return os.path.exists(self.path)
+
     @property
     def path(self):
         return os.path.join(OUTPUT_PATH, self.cert_id + '-cert.pub')

request.py

@@ -20,6 +20,9 @@ class SignRequest(object):
     def __repr__(self):
         return ( "%s %s" % ( str(self.__class__.__name__), str(self.req_id) ) )
 
+    def __bool__(self):
+        return os.path.exists(self.path)
+
     @property
     def name(self):
         raise NotImplementedError()