Remove redundant body validation for createBackup

The body validation is redundant for createBackup, should remove it. In
fact that this extension is registered as WSGI action implies that the
body must contain a key with the same name as action.

Change-Id: I327688b2df50782bc38b97a4750f83a607623ce4
Closes-Bug: #1254988
Removes disk-config extension from v3 api

This patch removes disk-config extension for v3 api, and also
remove related tests. And remove resize extension point for
servers core because there isn't any extension using it anymore
and there isn't good way to test resize extension point.

Implements bp api-v3-remove-disk-config
DocImpact

Change-Id: I894ae6f4e0d6c9f956a71e8e061ccb3fa1b039d8
Merge "Removes disk-config extension from v3 api"
Merge "Remove redundant body validation for createBackup"