Make the expected blessings a slice rather than a string

This is from a comment in a previous CL, that allowing the
app to specify a set of acceptable blessings will make the
system more flexible.

MultiPart: 4/4
Change-Id: Iab39512f322edabfeb163dde441b7970d1de7048
diff --git a/go/src/v.io/x/sensorlog/internal/client/device.go b/go/src/v.io/x/sensorlog/internal/client/device.go
index 106d649..81f4fe6 100644
--- a/go/src/v.io/x/sensorlog/internal/client/device.go
+++ b/go/src/v.io/x/sensorlog/internal/client/device.go
@@ -48,7 +48,7 @@
 	// really relevant until syncgroup Leave is implemented.
 	sgMemberInfo := wire.SyncgroupMemberInfo{SyncPriority: config.SyncPriority}
 	sgId := wire.Id{Name: devSgName, Blessing: "blessing"}
-	if _, err := db.SyncgroupForId(sgId).Join(ctx, sgPublishSb, "", sgMemberInfo); err != nil {
+	if _, err := db.SyncgroupForId(sgId).Join(ctx, sgPublishSb, nil, sgMemberInfo); err != nil {
 		return nil, err
 	}