Import missing Horizon SCSS dependencies

Horizon master renders firewalls.scss through ADD_SCSS_FILES, but resource_topology.scss no longer receives all of the variables and mixins it expects from that entry point. This causes asset compression to fail with an undefined $border-color while enabling neutron-fwaas-dashboard in the Horizon image.

Import dashboard variables and Bootstrap mixins before resource_topology.scss so the FWaaS stylesheet follows Horizon’s expected SCSS dependency chain.