Fix the broken security unit tests
Fix the broken security unit tests

Summary:
Since /var/facebook/krb5/krb5.keytab now defaults to use only
sys.<hostname_scheme> credentials, modify the test to use the same for service
identity.

This fixes the unit tests.

Test Plan: fbconfig -r thrift/lib/cpp2/test/ && fbmake runtests

Reviewed By: alandau@fb.com

Subscribers: alandau, bmatheny, mshneer

FB internal diff: D1616695

Tasks: 5356642

SergeyMakarenko/fbthrift,guker/fbthrift,kangkot/fbthrift,jesboat/fbthrift,project-zerus/fbthrift,uber/fbthrift,maxwell-demon/fbthrift,getyourguide/fbthrift,uber/fbthrift,getyourguide/fbthrift,kangkot/fbthrift,HankFaan/fbthrift,maxwell-demon/fbthrift,Orvid/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,Orvid/fbthrift,kangkot/fbthrift,raphaelamorim/fbthrift,HankFaan/fbthrift,chjp2046/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,kangkot/fbthrift,guker/fbthrift,project-zerus/fbthrift,LinusU/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,sinjar666/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,raphaelamorim/fbthrift,chjp2046/fbthrift,guker/fbthrift,sinjar666/fbthrift,uber/fbthrift,project-zerus/fbthrift,guker/fbthrift,project-zerus/fbthrift,uber/fbthrift,facebook/fbthrift,kangkot/fbthrift,project-zerus/fbthrift,jesboat/fbthrift,maxwell-demon/fbthrift,jesboat/fbthrift,getyourguide/fbthrift,sinjar666/fbthrift,raphaelamorim/fbthrift,HankFaan/fbthrift,raphaelamorim/fbthrift,LinusU/fbthrift,LinusU/fbthrift,HankFaan/fbthrift,LinusU/fbthrift,jesboat/fbthrift,Orvid/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,chjp2046/fbthrift,facebook/fbthrift,getyourguide/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,raphaelamorim/fbthrift,Orvid/fbthrift,raphaelamorim/fbthrift,getyourguide/fbthrift,soumith/fbthrift,soumith/fbthrift,kangkot/fbthrift,fengshao0907/fbthrift,jesboat/fbthrift,facebook/fbthrift,chjp2046/fbthrift,HankFaan/fbthrift,facebook/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,jesboat/fbthrift,jesboat/fbthrift,soumith/fbthrift,uber/fbthrift,chjp2046/fbthrift,maxwell-demon/fbthrift,maxwell-demon/fbthrift,SergeyMakarenko/fbthrift,soumith/fbthrift,HankFaan/fbthrift,raphaelamorim/fbthrift,HankFaan/fbthrift,raphaelamorim/fbthrift,LinusU/fbthrift,sinjar666/fbthrift,guker/fbthrift,sinjar666/fbthrift,soumith/fbthrift,guker/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,maxwell-demon/fbthrift,fengshao0907/fbthrift,fengshao0907/fbthrift,Orvid/fbthrift,soumith/fbthrift,HankFaan/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,HankFaan/fbthrift,getyourguide/fbthrift,jesboat/fbthrift,Orvid/fbthrift,guker/fbthrift,maxwell-demon/fbthrift,raphaelamorim/fbthrift,fengshao0907/fbthrift,uber/fbthrift,sinjar666/fbthrift,jesboat/fbthrift,kangkot/fbthrift,facebook/fbthrift,uber/fbthrift,fengshao0907/fbthrift,SergeyMakarenko/fbthrift,soumith/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,raphaelamorim/fbthrift,fengshao0907/fbthrift,project-zerus/fbthrift,SergeyMakarenko/fbthrift,sinjar666/fbthrift,fengshao0907/fbthrift,guker/fbthrift,jesboat/fbthrift,chjp2046/fbthrift,sinjar666/fbthrift,guker/fbthrift,uber/fbthrift,soumith/fbthrift,chjp2046/fbthrift,project-zerus/fbthrift,Orvid/fbthrift,chjp2046/fbthrift,raphaelamorim/fbthrift,uber/fbthrift,kangkot/fbthrift,project-zerus/fbthrift,kangkot/fbthrift,soumith/fbthrift,facebook/fbthrift,fengshao0907/fbthrift,LinusU/fbthrift,fengshao0907/fbthrift,jesboat/fbthrift,project-zerus/fbthrift,HankFaan/fbthrift,uber/fbthrift,Orvid/fbthrift,chjp2046/fbthrift,facebook/fbthrift,guker/fbthrift,maxwell-demon/fbthrift,LinusU/fbthrift,project-zerus/fbthrift,soumith/fbthrift,uber/fbthrift,project-zerus/fbthrift,sinjar666/fbthrift,fengshao0907/fbthrift,uber/fbthrift,chjp2046/fbthrift,maxwell-demon/fbthrift,fengshao0907/fbthrift,kangkot/fbthrift,maxwell-demon/fbthrift,project-zerus/fbthrift,LinusU/fbthrift,kangkot/fbthrift,soumith/fbthrift,maxwell-demon/fbthrift,jesboat/fbthrift,HankFaan/fbthrift,guker/fbthrift