roachtest: standardize start options for UA and non UA

More concretly, build `StartOpts` based on roachtest's implementation
of `DefaultStartOpts`, which includes creating a default backup
schedule on start.

Epic: none

Release note: None

pkg/cmd/roachtest/option/options.go

@@ -59,7 +59,7 @@ func DefaultStartSingleNodeOpts() StartOpts {
 // DefaultStartVirtualClusterOpts returns StartOpts for starting an external
 // process virtual cluster with the given tenant name and SQL instance.
 func DefaultStartVirtualClusterOpts(tenantName string, sqlInstance int) StartOpts {
-	startOpts := StartOpts{RoachprodOpts: roachprod.DefaultStartOpts()}
+	startOpts := DefaultStartOpts()
 	startOpts.RoachprodOpts.Target = install.StartServiceForVirtualCluster
 	startOpts.RoachprodOpts.VirtualClusterName = tenantName
 	startOpts.RoachprodOpts.SQLInstance = sqlInstance
@@ -71,7 +71,7 @@ func DefaultStartVirtualClusterOpts(tenantName string, sqlInstance int) StartOpt
 // DefaultStartSharedVirtualClusterOpts returns StartOpts for starting a shared
 // process virtual cluster with the given tenant name.
 func DefaultStartSharedVirtualClusterOpts(tenantName string) StartOpts {
-	startOpts := StartOpts{RoachprodOpts: roachprod.DefaultStartOpts()}
+	startOpts := DefaultStartOpts()
 	startOpts.RoachprodOpts.Target = install.StartSharedProcessForVirtualCluster
 	startOpts.RoachprodOpts.VirtualClusterName = tenantName
 	return startOpts