This commit add supports for secondary group of proxy
This way, a proxy can now be listed from another group like this :
<ProxyGroup name="new_group">
  <Proxy group = "group" name ="proxyname" />
</ProxyGroup>

Enabling easier dynamic list of proxy in proxy list domain.
The defaults StreamTracer source also changes because of
DefinitionManager handling porxy in a set ordered by proxy name,
so some tests needed to be updated.