Fix condition of Swift SLO API tests run

Swift Static Large Object API tests get run even if not set slo in
[object-storage-feature-enabled] of tempest.conf.
So it always fails unless Swift's slo is enabled.

This change makes the test run only when set slo in tempest.conf.

Change-Id: I9f256e0395fd606fd1cf64d2bffd5c00802c9cf4
Closes-Bug: 1458768
1 file changed