Support subnet lookup for app gateway

[l3ender]

SUMMARY

This PR updates the app gateway module so a subnet can be provided by name and virtual network name, as opposed to needing the full subnet resource ID.

This change is backward compatible and will complement existing playbooks which use subnet resource ID.

ISSUE TYPE

• Feature Pull Request

COMPONENT NAME

plugins/modules/azure_rm_appgateway.py

ADDITIONAL INFORMATION

Before:

- name: Create instance of Application Gateway
  azure_rm_appgateway:
    ...
    gateway_ip_configurations:
      - subnet:
          id: /subscriptions/xxxxxxxxx-xxxx-xxxx-xxxxxxxxxxxxxx/resourceGroups/myresourcegroup/providers/Microsoft.Network/virtualNetworks/myvnet/subnets/myvnet-appgateway-subnet
        name: app_gateway_ip_config

After:

- name: Create instance of Application Gateway
  azure_rm_appgateway:
    ...
    gateway_ip_configurations:
      - subnet:
          name: myvnet-appgateway-subnet
          virtual_network_name: myvnet
        name: app_gateway_ip_config

[Fred-sun]

@l3ender Thank you for your contribution. We will complete the review and push forward the merger as soon as possible! Thank you very much!

[Fred-sun]

@l3ender lines 705,706,707,740,741 continuation line over indented for visual indent! Thank you very much!

[l3ender]

@Fred-sun Updated, thank you!

[Fred-sun]

@l3ender Now that you have described the molecular parameters of the document, could you help define those parameters in args_spec? This keeps the document consistent with arg_spec. Thank you very much!