postgresql: move initdb to postgresql-setup

We shouldn't use sysvinit init script to initialize database when use systemd
as the init manager, so split initdb function to postgresql-setup.
Before starting postgresql server, we can use "postgresql-setup initdb" to
initialize the database cluster.

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>