Create filtering namespace for finding users and groups

[labkode]

In current CERNBox we have different namespaces to filters users and groups:

hugo will look into primary and e-groups
a:hugo will look into primary/secondary/service/e-groups

Given that we have now the possibility to share with lightweight accounts we should add a third option:
l:hugo to only look into lightweight accounts.

This prefixes need to be understood in our userprovider and groupprovider drivers so we query Grappa for what we need.