update MDS for unit server - work unit association
- now is possible to update the association between a work unit and an unit server(all can be updated also the unique id)

Signed-off-by: Claudio Bisegni <Claudio.Bisegni@lnf.infn.it>
max and min values are included, default value for instance can be 0 as the minumum value
update to new mongodb driver and ported the conf_file parameter to the common layer

Signed-off-by: Claudio Bisegni <Claudio.Bisegni@lnf.infn.it>
fixed username

Signed-off-by: Claudio Bisegni <Claudio.Bisegni@lnf.infn.it>
Added vas data area writeable file, and restructured some method through vfs class hierarchy

Signed-off-by: Claudio Bisegni <Claudio.Bisegni@lnf.infn.it>
scanned development

Signed-off-by: Claudio Bisegni <Claudio.Bisegni@lnf.infn.it>