take interval into account, improved client usage.

we only need to set one client for glance/nova when querying (was
setting for each tenant which made things slow).

Fixes #11: interval not being taken into account.
take interval into account, improved logging.

adds a new debug log function and config param, adds a verbose entry for
each plugin run for tracing (Fixes #10).

take interval into account (Fixes #11).
proper data for volumes, snapshots. take interval.

takes interval config into account (Fixes #11).

query volumes and snapshot data individually, and sum for each tenant.