Fixes EGI-FCTF/rOCCI-server#27, vnet attrs

When (re)adding new mix-ins to existing resource instances, corr.
attribute values are automatically re-set to `nil`. This has to be
handled manually.
Minor fix for storage description

Aligning behavior with virtual networks where summary has a default
used when filling in the description.
Merge pull request #34 from arax/one_vnet_tpl

Fixes EGI-FCTF/rOCCI-server#27, vnet attrs