data_smart: Don't use mutable objects as default args

data_smart: Don't use mutable objects as default args

These only have one instance created which means your subsequent datastores
can contain echos of previous ones. Obviously this is not the behaviour
we want/expect. It doesn't affect bitbake too badly as we only have one
datastore, it does massively potentially break our selftests though.

Thanks to Tim Amsell for pointing out the now obvious problem!

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>