Grant user to role

[Ma, Junjie]

Hi,

Currently, sentry only support grant group to role, there should be a 
reasonable feature to grant user to role. This is also the gap between Hive and 
Sentry, for Hive, the following command is supported:
        GRANT role_name TO USER user
I think it's an useful feature for authorization, and the SENTRY-711 is created 
for this. You can get the design doc, patch, review board's link in this JIRA.
Feel free for any comments, thanks.

Best regards,

Colin Ma(Ma Jun Jie)