Log translation hint for nova.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

Change-Id: I128da6cbb2be1b5b51b4a79fb38b3bf8206431d9
Merge "Log translation hint for nova.api"