Faster get_attrname in nova/objects/base.py

get_attrname function is called very very often,
and it's string concatenation method is not fast.

* Changing the string concatenation to a 2 times
  faster + operator instead of the % operator.

Change-Id: I326fd6dda8dd41a90757c69867afd62063231362
Merge "Faster get_attrname in nova/objects/base.py"