Log translation hint for Heat.api

Currently, Log translation is motivated by oslo's move to prioritized
translation of strings, as documented at
https://wiki.openstack.org/wiki/LoggingStandards#Log_Translation

- add log translation hints for warning, error and info levels
- move from LOG.warning to LOG.warn
- remove use of % as a string formatter, use the log functionality
  instead

Partial implements blueprint log-translation-hints

Change-Id: Ie17871f42f75085c2ac6a14fb1526df7bc3a9603
Merge "Log translation hint for Heat.api"