Support simple string in configuration

Allow parsing:
 - a string

openntpd:
  listen: '*'

 - a list of string

openntpd:
  server:
    - 192.168.0.1
    - 192.168.0.2

 - a list of dict

openntpd:
  server:
    - address: 192.168.0.1
      weight: 1
    - address: 192.168.0.2
      weight: 2
      rtable: 1

 - a list of (string | dict)

openntpd:
  server:
    - 0.pool.ntp.org
    - 1.pool.ntp.org
    - address: 192.168.0.1
      weight: 2