Fix: If dangling_timeout was set to 0 and auto_import_dangled

was set to yes, dangling indices were deleted by mistake,
because a RemoveDanglingIndices runnable was added
to every dangling indices, without considering the auto_import_dangled
setting.
Maybe it would be good to have a test where this timeout is set to '0' as here and also a tests to check that the functionality is correct when the timeout is non-zero? We could do this by using the randomBetween* methods in ElasticsearchTestCase to choose appropriate values randomly to test?