Make bin/swift testable part 1

To make bin/swift importable for testing it needs to be renamed/moved.
The final idea is to move it to swiftclient/shell.py and add only a stub
in bin/swift to import swiftclient/shell.py.

However, doing this in a single step makes reviewing very hard, because
now there are > 1400 lines deleted from bin/swift and added to
swiftclient/shell.py - Gerrit doesn't detect the moved file.

For example: https://review.openstack.org/#/c/73710/2/

This patch first moves the code to swiftclient/shell.py and uses
setup.py/cfg to create the stub file in bin/swift. A follow up
(https://review.openstack.org/#/c/76489/) will then add the stub itself
in bin/swift (and remove the entry in setup.py).

The change to tox.ini is related to bug 1290562 and can be removed in
the future.

Change-Id: Id86392ccaf09e1d80a5d808fb49827d6d18c2588