Security groups may be unavailable

Currently all missing security group situations are reported as errors
and exceptions. This is not the case - any user requesting a wrong id
or name can cause it and it's not an issue in Nova. A busy system can
generate a large number of those messages causing unnecessarily high
error count.

This change stops reporting this situation as exception in the logs.

Change-Id: Ide856cbbde2db025b0e2def79652cc6c9b00566b
Merge "Security groups may be unavailable"