az network _subgroup_ list behavioural differences

### Description
**Outline Issue Here**
There are inconsistent behaviours/requirements across the **az network _subgroup_ list** operations. If possible, please align the outliers with the more common behaviour.

**Issue/Concern/Improvement Request**
These subgroups require --resource-group for list operations:
local-gateway : Manage local gateways.
vnet-gateway : Use an Azure Virtual Network Gateway to establish secure, cross-premises connectivity.
vpn-connection : Manage VPN connections.

**Expected behaviour**
**az network _subgroup_ list** for above subgroups should dump all entries in the active subscription, as the subgroups below do.

The following subgroups happily dump all entries in the active subscription when **az network _subgroup_ list** is issued:
application-gateway: Manage application-level routing and load balancing services.
asg : Manage application security groups.
express-route : Manage dedicated private network fiber connections to Azure.
lb : Manage and configure load balancers.
nic : Manage network interfaces.
nsg : Manage Azure Network Security Groups (NSGs).
public-ip : Manage public IP addresses.
route-filter : (PREVIEW) Manage route filters.
route-table : Manage route tables.
vnet : Manage Azure Virtual Networks.
watcher : (PREVIEW) Manage the Azure Network Watcher.
traffic-manager profile : Manage the routing of incoming traffic.
dns zone : Manage DNS domains in Azure.

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### Environment Summary
**Install Method**
apt-get per instructions at https://docs.microsoft.com/en-us/cli/azure/install-azure-cli-apt?view=azure-cli-latest

**CLI Version**
$ az --version
azure-cli (2.0.21)

acr (2.0.15)
acs (2.0.19)
appservice (0.1.20)
backup (1.0.3)
batch (3.1.7)
batchai (0.1.3)
billing (0.1.6)
cdn (0.0.10)
cloud (2.0.10)
cognitiveservices (0.1.9)
command-modules-nspkg (2.0.1)
component (2.0.8)
configure (2.0.12)
consumption (0.1.6)
container (0.1.13)
core (2.0.21)
cosmosdb (0.1.15)
dla (0.0.15)
dls (0.0.18)
eventgrid (0.1.5)
extension (0.0.6)
feedback (2.0.6)
find (0.2.7)
interactive (0.3.11)
iot (0.1.14)
keyvault (2.0.14)
lab (0.0.13)
monitor (0.0.12)
network (2.0.18)
nspkg (3.0.1)
profile (2.0.15)
rdbms (0.0.9)
redis (0.2.10)
reservations (0.1.0)
resource (2.0.18)
role (2.0.14)
servicefabric (0.0.6)
sql (2.0.15)
storage (2.0.19)
vm (2.0.18)

Python location '/opt/az/bin/python3'
Extensions directory '/home/lwatterworth/.azure-AzureCloud/cliextensions'

Python (Linux) 3.6.1 (default, Nov 10 2017, 16:23:17)
[GCC 4.8.4]

Legal docs and information: aka.ms/AzureCliLegal

**OS and Version**
$ uname -a
Linux BB2469515907 4.4.0-101-generic #124~14.04.1-Ubuntu SMP Fri Nov 10 19:05:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

**Shell type and Version**
$ bash --version
GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu)