tox: update swift version to 2.1.0

The Swift 2.1.0 includes the header capitalization patch
https://review.openstack.org/93780/, which is required for Swift3.

Let's use the version for development.

Change-Id: I2b49ca8362234e1d74f44cdb90ccb551fb31fc45
Merge "tox: update swift version to 2.1.0"