Make OMP_NESTED false by default

[bartgol]

Since Kokkos 'usually' does not use this feature, it makes sense to disable it by default, so that a distract user does not leave it on in OpenMP by mistake. I'm not 100% sure, but there may be some performance hit in leaving it on. Either way, it makes more sense to have a feature not commonly used off, unless the user specifically asks for it.