Allow Salt policy to create admin-level tokens

Summary:
To allow a self-service token generation in Complector, allows the
Salt primary policy to issue tokens for the admin role.

Ref T1975.

Test Plan: Issue an admin token with salt-primary policy

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1975

Differential Revision: https://devcentral.nasqueron.org/D3355