Make the object storage section less Ubuntu centric

The Swift section of the install guide tends to assume that the user
uses Ubuntu. This change provide more information for other
distributions.

backport: havana
Closes-Bug: #1274473
Closes-Bug: #1274405
Change-Id: I1c1fbea43d29724067185e3ab261cd20b2e0ed72
(cherry picked from commit 5427452057c5719bf7cadf1bd768958d9cdab8c0)