New tenant name could be "default"

[LiuBodong]

this discussion is from issue #6220

[lenboo]

• I think the 'default'(value=-1) cannot be removed.

You should must select one tenant when saving DAG if 'default' is removed.
I don't think this operation is as simple as 'default '.
using 'default', the same DAG can be run different tenants when multiple users run one DAG.

• Maybe we can change the name 'default' to another one.