Removes XML namespace definitions from V3 API plugins

Removes the XML namespace definitions from the V3 API
plugins declarations. Deliberately avoids removing
the XML namespace definition from admin_actions to avoid
yet another rebase of the admin_actions split series.

Fixes up associated tests and requirement for V3 API extensions
to define a namespace property.

Partially implements blueprint remove-v3-xml-api

Change-Id: If3e1314cefdc1abcdd5eca44bcc2282cac664f05
Merge "Removes XML namespace definitions from V3 API plugins"