This patch makes the creation of the initial tenant & timeline optional.
The motivation was https://github.com/neondatabase/neon/pull/3905#discussion_r1153627388
However, most existing tests require the env.initial_tenant.
We don't want to change all of these here.
So, the apporach taken here is to make NeonEnv a subclass of
NeonEnvWithoutInitialTenant. It's not a proper subclass, but
more of wrapper type. Hence the __new__ hackery.
I guess this is a bad substitute for a Rust newtype + Deref impl.
The initial tenant & timeline are created in NeonEnv, and the
tenant is set as default (`--set-default`).
We rely more on that default than before. Specifically,
all neon_local invocation that previously used
"--tenant-id", (tenant_id if tenant_id is not None else env.initial_tenant)
now do
*(["--tenant-id", tenant_id] if tenant_id is not None else [])
The only real trouble with that was `Postgres` class's `pgdata_dir` member.
I solved this by making `neon_local pg create` return the PGDATA dir
path, instead of pre-computing it in Python. A net win, I think.
6e4d8e0b1c