roachprod: fix `roachprod init`.

[renatolabs]

When a cluster is started with the --skip-init option, the caller can run roachprod init at any time to initialize the cluster. Unfortunately, the code used to initialize the cluster was duplicated: one copy existed in the start path, and another in the init path. Since the latter is used far less frequently, it had a bug that went unnoticed: it hardcoded the first node index as 0, when node indices start at 1.

This commit fixes the issue by updating the constant and sharing code between init and start.

Fixes #88226.

Release note: None

[cockroach-teamcity]

This change is 

[renatolabs]

bors r=srosenberg,erikgrinaker

TFTR!

[craig[bot]]

Build succeeded:

• Bazel Essential CI (Cockroach)

[yuzefovich]

I think this change might have had an unintended side effect - we no longer set the test license, which causes failures like #88665.