Support for TLS client certificate authentication

This adds --tls-client-cert and --tls-client-key parameters and enables use
of that certificate/key pair when connecting to https servers.

internal/backend/azure/azure_test.go

@@ -16,7 +16,7 @@ import (
 )
 
 func newAzureTestSuite(t testing.TB) *test.Suite {
-	tr, err := backend.Transport(nil)
+	tr, err := backend.Transport(nil, "", "")
 	if err != nil {
 		t.Fatalf("cannot create transport for tests: %v", err)
 	}