pageserver: use identity file as node id authority and remove init command and config-override flags (#7766)

Ansible will soon write the node id to `identity.toml` in the work dir
for new pageservers. On the pageserver side, we read the node id from
the identity file if it is present and use that as the source of truth.
If the identity file is missing, cannot be read, or does not
deserialise, start-up is aborted.
 
This PR also removes the `--init` mode and the `--config-override` flag
from the `pageserver` binary.
The neon_local is already not using these flags anymore.

Ansible still uses them until the linked change is merged & deployed,
so, this PR has to land simultaneously or after the Ansible change due
to that.

Related Ansible change: https://github.com/neondatabase/aws/pull/1322
Cplane change to remove config-override usages:
https://github.com/neondatabase/cloud/pull/13417
Closes: https://github.com/neondatabase/neon/issues/7736
Overall plan:
https://www.notion.so/neondatabase/Rollout-Plan-simplified-pageserver-initialization-f935ae02b225444e8a41130b7d34e4ea?pvs=4

Co-authored-by: Christian Schwarz <christian@neon.tech>

Dockerfile

@@ -93,13 +93,14 @@ COPY --from=pg-build /home/nonroot/postgres_install.tar.gz /data/
 
 # By default, pageserver uses `.neon/` working directory in WORKDIR, so create one and fill it with the dummy config.
 # Now, when `docker run ... pageserver` is run, it can start without errors, yet will have some default dummy values.
-RUN mkdir -p /data/.neon/ && chown -R neon:neon /data/.neon/ \
-    && /usr/local/bin/pageserver -D /data/.neon/ --init \
-       -c "id=1234" \
-       -c "broker_endpoint='http://storage_broker:50051'" \
-       -c "pg_distrib_dir='/usr/local/'" \
-       -c "listen_pg_addr='0.0.0.0:6400'" \
-       -c "listen_http_addr='0.0.0.0:9898'"
+RUN mkdir -p /data/.neon/ && \
+  echo "id=1234" > "/data/.neon/identity.toml" && \
+  echo "broker_endpoint='http://storage_broker:50051'\n" \
+       "pg_distrib_dir='/usr/local/'\n" \
+       "listen_pg_addr='0.0.0.0:6400'\n" \
+       "listen_http_addr='0.0.0.0:9898'\n" \
+  > /data/.neon/pageserver.toml && \
+  chown -R neon:neon /data/.neon
 
 # When running a binary that links with libpq, default to using our most recent postgres version.  Binaries
 # that want a particular postgres version will select it explicitly: this is just a default.
@@ -110,3 +111,6 @@ VOLUME ["/data"]
 USER neon
 EXPOSE 6400
 EXPOSE 9898
+
+CMD /usr/local/bin/pageserver -D /data/.neon
+