all services: use ConfigItem instead of deprecated ConfigurationItem

The class `ConfigurationItem` from AstroPy is deprecated and will be removed in version 1.0
Actually for lists this might create a reference instead of a copy ... I'll try to watch out for that ... this also seems OK though:
```
In [22]: vizier.conf.server = 'vizier.china-vo.org'

In [23]: v1.VIZIER_SERVER
Out[23]: u'vizier.u-strasbg.fr'
```