Allow edit of user role

I think it would be helpful to allow changing and updating a user's role from horizon

root@tigris01 ~(keystone_admin)]# keystone help |grep role
    role-create Create new role.
    role-delete Delete role.
    role-get Display role details.
    role-list List all roles.
    user-role-add Add role to user.
    user-role-list List roles granted to a user.
    user-role-remove Remove role from user.
    bootstrap Grants a new role to a new user on a new tenant, after
[root@tigris01 ~(keystone_admin)]# keystone help user-role-add
usage: keystone user-role-add --user <user> --role <role> [--tenant <tenant>]

Add role to user.

we can actually user role-delete + role-create or role-create + --role <role>, --role-id <role>, --role_id <role>