Fix the duplicated extension summaries

If operating `nova --os-compute-api-version 3 extension-list`, the
summaries of both flavor-extra-specs and flavors extensions are the
same "Flavors Extension.". The summary of flavor-extra-specs should
include its feature.
There is a same issue on os-extended-server-attributes and
os-extended-availability-zone extensions also.

This patch fixes these summaries.

Change-Id: I6778229221ca80632b3a303745ff5133488f327d
Related-Bug: #1318973
Merge "Fix the duplicated extension summaries"