Do not enable swift when radosgw is used

RadosGW doesn't totally work as swift replacement in the context of
tempest, see https://bugzilla.novell.com/show_bug.cgi?id=891306

Therefore, for now, we restrict the swift tests to the case where swift
is deployed.
Merge pull request #109 from vuntz/no-radosgw

Do not enable swift when radosgw is used