Example files for Modify User Attributes API call

At present, when a Cloud Database user creates a username, password and hostname for a database user,
he does not have the ability to modify these user attributes.
This API enables the Cloud DB user to make changes to one or more of these user attributes (username,hostname,password).
This commit includes the addition of relevant example JSON and XML request-repsonse files

Implements: blueprint modify-userattributes