Add cinder volume list support

Manage SL portable storage volumes. Those volumes are used for VSI
deploy(boot/swap) or attachable/detachable to the running VSI.

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.

Add volume unittest coverage

Change-Id: Iba60d56ac67d82e9227f654d8e5ef907f2cd1158
Merge pull request #114 from ajiang38740/sl_portable_storage_cinder_api

Add cinder volume list support