Remove usage of self.__dict__ for message var replacement

This is the same as using locals() for variable replacement in a log
message which violates hacking rule H501.  On Cinder this actually
caused the cinder-api process to hang on python 2.6.

Nova has the same code so this fixes the hacking violation.

Note that the H501 rule was updated in commit 55e96ee but it's not in a
released version of hacking yet (and won't be in time for Juno).

Closes-Bug: #1365901

Change-Id: I1be0db327a2fa1e866328eecd2e7c3dd3ee3cc93
Merge "Remove usage of self.__dict__ for message var replacement"