WIP: 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?
"user-admin" is a tag not a username. Given the rest of the API is (currently) dealing with usernames, it's probably worth using usernames in the error message too.

There is an ongoing discussion about whether the usermanager API should use tags or usernames so this might all change (@howbazaar has more detail) - but for now, I would keep things consistent, one way or the other.