Add compatibility code for handling new options in existing proposals

Without this, the barclamp will use "true" as value for
auto_assign_floating_ip when displaying pre-existing proposals, and will
actually crash because
@proposal.raw_data['attributes'][@proposal.barclamp]["scheduler"] is
nil.

This is not really optimal as it hard-codes the defaults also in the
view of the barclamp, but it's only use for pre-existing proposals.