This commit adds reserved or built-in user and role support to x-pack. The reserved roles
cannot be modified by users. The reserved users also cannot be modified with the exception
of changing the password for a user.
In order to change the password for a user, a new API has been added. This API only supports
changing passwords for native and reserved users.
To support allowing a user to change their own password, a default role has been added to grant
access. This default role only grants access to user operations that pertain to the user that is
being authorized. In other words, the default role grants `joe` the ability to change their own password
but does not allow them to change the password of a different user.
Additionally, the authenticate API was made a transport action and is granted by the default role.
Closeselastic/elasticsearch#1727Closeselastic/elasticsearch#1185Closeselastic/elasticsearch#1158
Original commit: elastic/x-pack-elasticsearch@1a6689d90f
jaymode
Tanguy Leroux