Fix record extraction for long running VMs

Commit 0058b6c introduced a bug in how we retrieve the records, since we introduced the "deleted" filter and this was filtering out all the instances that are not deleted.

In order to get accurate results, accounting records must be regenerated at sites, possibly month by month, like:

caso-extract  --extract-from 2017-01-01 --extract-to 2017-01-31
caso-extract  --extract-from 2017-02-01 --extract-to 2017-02-28
(...)

Bugfixes:

• #47 cASO only accounts for deleted VMs bug
• #44 tenant or project in voms.json documentation