Add cinder volume list support

The motivation for this change set is to start the cinder volume APIs support under 
jumpgate. The current supported storage backend is SL portable storage device(Virtual_Disk_Image).

SL portable storage device is the disks that is used by VSI deploy today. It can also be attached/detached
after deploy via hotplug. This covers most of the volume lifecycle management(which is better than SL iSCSI
and it cannot attach/detach automatically). 

The limitation for SL portable storage device is limited to 5 total per VSI by SL. In addition, it doesn't
support snapshot. 

The current pull request includes the following items:
1. jumpgate volumes framework and endpoints.
2. jumpgate volume list/show of SL portable storage
3. jumpgate volume unittest for list/show

I will have another set of changes soon to include:
1. jumpgate volume create
2. jumpgate volume delete
3. jumpgate volume attach
4. jumpgate volume detach
5. jumpgate volume type list(use jumpgate.conf and will support local vs san and availiability_zone)

Notes:
Also rename jumpgate/cmd.py to jumpgate/cmd_main.py due to the
namespace collision of cmd.py with the python module cmd used by
pdb.