Jointly load ExternalNetwork with Network

Jointly load ExternalNetwork with Network

Bug 1177572

This patch leverages the same approach as bug 1174111 for avoiding
loading ExternalNetwork element from db for each network element.
make_network_dict, _extend_network_dict_l3 are refactored accordingly.

Also, this patch sligthly alters the logic of process_l3_network_create
and process_l3_network_update as these methods assumed
_extend_network_dict_l3 was always executed before retuerning.

Finally, calls to get_network and get_networks in plugins which only
extended the response with l3 attributes have been completely removed.

Change-Id: I3b4c28ec0c78f731a534c2b66150d529fd797e72