[TEST] wait for yellow after index was created in BWC tests
[TEST] Make default flush opertion blocking
[TEST] Pick random version from backwards directory

The upgrade tests required to specify the lower and the upper version
that it should run against. This commit adds support for random picks
if either the lower or the upper or both are not specified.
Test: finish all merges before test returns to side step 'Delete Index failed - not acked' failure