Fix `grafana_user` and switch to using `flush`

[alexjfisher]

Previously save_user was being called once per changed property and calls were also being made to update user properties, password and the admin flag regardless of whether these properties needed updating.

Using flush is more efficient.

• full_name is fixed (it was previously a parameter instead of a property).
• When a user is created, is_admin is correctly set in a single Puppet run.
• Properties can be managed individually.
• Instead of making password mandatory, (when creating a user), if password isn't specified, a random one is used.

Fixes #121