Write postgres options to postgresql.conf

Originally we were passing postgresql options as an argument of `pg_ctl
start`. It was nice and convenient because doesn't require to touch
configuration files but this method has one significant drawback: it
wasn't possible to change values of options which were passed as an
arguments without restart (event for the case when option reqires only
reload). Instead of doing that (passing options as arguments) we will:
1) rename original postgresql.conf to postgresql-base.conf
2) write options into postgresql.conf which has `include
  'postgresql-base.conf'` on the the third line after comment that this
  file is generated by Patroni and you should not change it manually
3) listen_addresses and port are still passed as an arguments to the
  pg_ctl (just to be foolproof against ALTER SYSTEM set port to 'random')

In addition to that this commit makes some attributes of `Postgresql`
class private (prefixes them with _)