Change the default value of vo_property to VO

[enolfc]

Description

To avoid creating a different property for every tool out there that deals with VOs (cloud-info-provider, fedcloudclient, cloudkeeper), use a generic VO property that can fit all of them and not just the accounting.

[alvarolopez]

Although I would agree, I think that we should name it something else, like "EGI:VO" or something like that. VO seem to generic to me.