Add fence_mode to network model

fence_mode can be 'isolated' (disconnected from EdgeGateway) or
'natRouted' (connected to NatService on EdgeGateway).

The network types differ significantly enough to have a separate
set of tests, to be expanded later.
Ensure get_network_complete mock returns like API

Booleans were being passed through unaltered. The API
returns them as strings, and in Real mode we do not post-process
them into boolean types.

Update so that booleans are returned as strings, as per
Real mode.
Add is_shared attribute to network Model

is_shared reports whether a network is shared across all
vDCs within an organization, or just within the vDC it is defined in.
Merge pull request #2712 from alphagov/vcd_network_model_extras

[vcloud_director] network model fence_mode and ip_shared attributes