Add attributes that are VO and site dependent
Sometimes VOs need to specify attributes that are VO specific may be different depending on the site.
This behavior should be discouraged because the VOs should not worry about where they run (besides receiving the resources they need), but some may be difficult to avoid (e.g. a project ID, billing code, ...).
These attributes could be still specified in the client (Frontend) but be a dictionary of values keyed using the entry or entry_set ID (would take precedence the entry, entry_set, the first one encountered if it is a sorted dict? - needs to be specified).
If there is a default value this should be used if the entry is not matching any of the keys, otherwise, the attribute will be undefined on that entry.
Should there be also Factory attributes keyed using the client/group ID?
A possible implementation could add a new attr flag (is_dictionary) and use string dictionaries (key:val, ... , functions are in singularity_lib.sh)
The factory would do the resolution for parameters attributes, an add_config_line equivalent would do the resolution for file transmitted attributes. Probably it should be a different param file, the entry_set name should be known to the glidein.