added apiserver and api client implementation of change password.

Added the api server and client set password command

Added the change-password user command

Thing to consider: 
> SetPassword only lets you change the password of the currently logged in user. Should the api really force this or should the commands force this behaviour (but not allowing the user to be specified)

> The change-password command writes the new password back to the yaml. Maybe the new file should be output as well?
That's done for consistency with older versions of the juju client which only sent the Tag field, which actually contained a username, not a User tag. I'm ok with you doing the same for SetPassword just to be consistent (not that any client will actually be sending anything in Tag).