fixed sentinel policy

[Andrew-Klaas]

[hashicorp-cla]

All committers have signed the CLA.

[rberlind]

One more important comment. I believe valid_principals could be a string containing multiple valid principals. So, checking equality on this is probably not right. Instead, you should check if identity.entity.aliases[0].name is in request.data.valid_principals. You could do this with request.data.valid_principals contains identity.entity.aliases[0].name