reflow/tls: use certs from ticket

Summary:
Currently, we generate certs per user. It is inconvenient to be able to use reflow (querying) from other machines unless you copy the certs file out everywhere.
I have also released a new grailbootstrap 0.2
This change doesn't require the OSS bootstrap to be updated.

Test Plan: Ran a test program and it succeeded.

Reviewers: marius, smahadevan, jcharumilind

Reviewed By: marius

Differential Revision: https://phabricator.grailbio.com/D38246

fbshipit-source-id: ae4bcda

tls/authority.go

@@ -36,6 +36,11 @@ var (
 	driftMargin = time.Minute
 )
 
+// Certs defines an interface used to obtain TLS client and server certificates.
+type Certs interface {
+	HTTPS() (client, server *cryptotls.Config, err error)
+}
+
 // Authority is an infrastructure provider that implements a TLS
 // authority, capable of issuing TLS certificates. Its implementation
 // requires that a file be specified (through the flag parameter