Follow-up of https://github.com/neondatabase/neon/pull/3270 which made
an example from main README.md not working.
Fixes that, by adding a way to specify a default tenant now and modifies
the basic neon_local test to start postgres and check branching.
Not all neon_local commands are implemented, so not all README.md
contents is tested yet.
For every Python test, we start the storage first, and expect that
later, in the test, when we start a compute, it will work without
specific timeline and tenant creation or their IDs specified.
For that, we have a concept of "default" branch that was created on the
control plane level first, but that's not needed at all, given that it's
only Python tests that need it: let them create the initial timeline
during set-up.
Before, control plane started and stopped pageserver for timeline
creation, now Python harness runs an extra tenant creation request on
test env init.
I had to adjust the metrics test, turns out it registered the metrics
from the default tenant after an extra pageserver restart.
New model does not sent the metrics before the collection time happens,
and that was 30s before.
