Add ability to download without writing to disk.
Add ability to download without writing to disk.

This patch adds a "--no-download" option to the "download" command.
When given, all writing to disk is bypassed, while still actually
downloading the data and validating etags.

This can be handy when you're using the swift command-line client to
test out a swift cluster and don't want client-side disk writing to be a
bottle-neck (but you still want to know about any etag validation
failures).

Change-Id: I0a511f473a64820161d1eb529b995900742794f2

JioCloud/python-swiftclient,iostackproject/IO-Bandwidth-Differentiation-Client,varunarya10/python-swiftclient,krnflake/python-hubicclient,pratikmallya/python-swiftclient,citrix-openstack-build/python-swiftclient,varunarya10/python-swiftclient,cbrucks/Federated_Python-Swiftclient,citrix-openstack/build-python-swiftclient,zackmdavis/python-swiftclient,citrix-openstack-build/python-swiftclient,sohonetlabs/python-swiftclient,zackmdavis/python-swiftclient,iostackproject/IO-Bandwidth-Differentiation-Client,pratikmallya/python-swiftclient,VyacheslavHashov/python-swiftclient,openstack/python-swiftclient,VyacheslavHashov/python-swiftclient,jeseem/python-swiftclient,ironsmile/python-swiftclient,openstack/python-swiftclient,citrix-openstack/build-python-swiftclient,jeseem/python-swiftclient,ironsmile/python-swiftclient,sohonetlabs/python-swiftclient,JioCloud/python-swiftclient,cbrucks/Federated_Python-Swiftclient,cbrucks/Federated_Python-Swiftclient,citrix-openstack/build-python-swiftclient