Parse "networks" attribute if loading os-networks

"networks" attribute of "create a server" API should be used when
Neutron/os-network extension is loaded as TODO which is removed with
this patch mentions. V2.1 os-network extension has been implemented
with I94b7b476cbb02725a396725a379417b64afd58a7 already, so this patch
enables "networks" attribute for os-network extension.

Partially implements blueprint v2-on-v3-api

Change-Id: Ibe59ddc53d074f5b8147e24b35066d38d73c7c33
Merge "Parse "networks" attribute if loading os-networks"