API for Modify User Attributes - username, host, 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 make changes to one or more of these user attributes (username,hostname,password).
These changes are in the troveclient to enable simple command line requests

Implements: blueprint modify-userattributes

Change-Id: I0887f2d927eec8c77c0f562c9d855a370591231a