CIMI: add unit tests for system machines and volumes
Fix empty network_interfaces in Mock#destroy_instance
CIMI Systems: fix for DTACLOUD-559 - id was not set for SystemMachineCollection and SystemVolumeCollection

https://issues.apache.org/jira/browse/DTACLOUD-559
https://github.com/deltacloud/deltacloud-core/pull/28
https://github.com/deltacloud/deltacloud-core/pull/27