1. Improve _validate_external_subnet to get the result directly
from DB instead of a get_networks api call
2. Skip configuration validation in _is_overlay_network as the type
of TZ is validated at plugin init anyway
3. Some nit changes in create_subnet
4. Improve is_ddi_supported to get the network if it was
already retrived
5. Improve _get_net_tz to skip backend access when possible
Change-Id: I24aed399a478e2d391ab7b45fe38e53b77116abb
253273ca5f