Adding shared flag support to network v2 list api

As spec defined [0], jumpgate should care about shared flag for network
v2 list api. And currently horizon use this flag to obtain shared and
non-shared network by two separted calls and orignize network list in
GUI [1], so without this fix horizon will get wrong as well.

[0] http://developer.openstack.org/api-ref-networking-v2.html
[1]
https://github.com/openstack/horizon/blob/master/openstack_dashboard/api/neutron.py#L535

Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>