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
Merge "Jointly load ExternalNetwork with Network"