Separate rbac calculation from _make_network_dict

Separate rbac calculation from _make_network_dict

When a subnet dict was being created, it was calling
_make_network_dict to get the 'shared' flag for the
subnet. The issue with this is that the _make_network_dict
function would iterate over the subnets on the passed in
network object, which would trigger a database lookup
of all of the subnets.

This patch just separates the 'shared' flag calculation out
into a separate function that both calls can leverage.

Change-Id: I2cb766ce1fd8ddcc75209f9e92221a3b77015ea2
Closes-Bug: #1525295
Partial-Bug: #1513782