LVM Thin volumes support added to nova-volume and volume error-deleting problem solved

PCI-1576:
Add LVM thin volumes support to nova-volume:
- utils.py: function for converting size in human readable format to bytes added
- driver.py: new volume is created within the thin pool if the LVM thin support is enabled

PCI-1718:
- iscsi.py: solves the volume error-deleting problem (when the volume is not created for example due to overcommit limit exceeded or insufficient disk space)