check properly if a config attribute value is not a boolean

I don't know if this commit is related to or fixes any bugs, but
there are many bugs reported about fog / shroud / mp campaigns it
seems, and these lines always looked a bit squirrely to me. Since
boolean config values can be indicated by "yes", "no", "true" or
"false", testing for the presence of "yes" / "no" only seems like
it won't work in general, even if it works oftentimes. This commit
replaces it with a test for whether the to_bool function is
selecting the default boolean value.