Modify User Attributes - name, host and password

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 modify one or more of these user attributes (username,hostname,password).

Implements: Blueprint modify-userattributes

Change-Id: I253cacf998e93b3d6136905dbb4a3516272efb0e
Merge "Modify User Attributes - name, host and password"