Adds XML namespace example for disk config extension

Adds XML namespace and specifying of the disk config value
to the disk config extensions api sample test data for server
creation. This is being done so the api samples used by the
documentation contain the example of this usage.

DocImpact

Change-Id: Icd7aa3743f71dd1cb8575605e5f1ce5da6bd37bf
Closes-Bug: 1189817
Merge "Adds XML namespace example for disk config extension"