Add python-swift client to the requirements

Since Id4b27ee70f420f7536b4c698eb1f3bba15dbd1e8 we have swift scenarios tests
which means that we must include swiftclient as a requirement.
Closes-bug: #1265936

Cherry-picked from Iee233867446cce098dbce9e5ab9ab1d4b19d97b3

Change-Id: I6d2ceb62aacb84f183cd8037e5b99cc5666b6f67