Quota management

[livenson]

It should be possible to limit resource usage on vcdm.

Implementation idea

For each user add limit on a resource type (as a separate field in user directory). When issuing modification requests, check if a new request would violate constraint.

Resource types:

• total size
• total cost of data

Concern

Need to differentiate roles. Doesn't make sense for common users setting their own quotas. Probably would make sense to create a standalone set of tools for administrating vcdm deployment, i.e. modifying data store not through CMI