Add script postgres_node_init to initialize Postgres cluster node

A master or slave can be initialized easily with a simple command for
a master like that:
postgres_node_init -p 5432 -D $PGDATA
For a slave:
postgres_node_init -p 5433 -D $PGDATA_SLAVE -q 5432 -s

This is more flexible than the current structure using pg_start and
will be replaced by it soon. This only does initialization, but supports
replication cascading naturally.