Add spec for app-agnostic-logging-parameters

Describe changes to openstack/common/context.py to use it as
a base class for request contexts in the other apps and to
have a standardized set of fields in the logging format
string that are not tied to the application.

Change-Id: I1850c081c1f4a752fd73d9b7a9c86bd60c2b7cd2
Merge "Add spec for app-agnostic-logging-parameters"