Add master tablet type in addition to replica as default source tablet types

Signed-off-by: Rohit Nayak <rohit@planetscale.com>

go/vt/vttablet/tabletmanager/vreplication/engine.go

@@ -72,7 +72,9 @@ func init() {
 	withDDL = withddl.New(allddls)
 }
 
-var tabletTypesStr = flag.String("vreplication_tablet_type", "REPLICA", "comma separated list of tablet types used as a source")
+// this are the default tablet_types that will be used by the tablet picker to find sources for a vreplication stream
+// it can be overridden by passing a different list to the MoveTables or Reshard commands
+var tabletTypesStr = flag.String("vreplication_tablet_type", "MASTER,REPLICA", "comma separated list of tablet types used as a source")
 
 // waitRetryTime can be changed to a smaller value for tests.
 // A VReplication stream can be created by sending an insert statement