Added templates for feature configuration description/reference

Added templates for feature configuration description/reference

The featureconfig.rst document should be used to describe the configuration of
 the <XYZ> feature.
 The content will be added to the Brahmaputra configuration guide which intends
to describe how to configure the Brahmaputra release to a stable and ready state
 For feature projects this should refer to the use of deployment tool scenario
 selections and post deployment configuration specific to the feature.
Also added is the postinstall.rst document for features to describe how to
check and analyze the result of test cases for respective feature.

Change-Id: Idf700026792efb2dfd9b72fe5dd49e605e41a84a
Signed-off-by: Sofia Wallin <sofia.wallin@ericsson.com>
(cherry picked from commit c7f20d6f115677bd3965d484667eb654be5104c9)