tests: fix MultiStrOpt value

Last oslo.config was magically converting MultiStrOpt "messaging" to ["messaging"],
that was not expected, oslo.messaging have a test that use "messaging"
instead of a list.

This change fixes that.

(cherry picked from commit 4818fda4)
Change-Id: I65b8cc551dde6f80d979640ba3db097572f3fe9b