feat: allow using existing subnet

[brianbyers]

customer needed the ability to reuse an existing network/subnet since they don't allow random ones to be created since they require planning for each vnet that is created internally. Made the network pieces optional and updated any references.

[lacework-code-security]

(Audit Mode) Lacework IAC found 5 new violations

Violation	Severity	File	Guidelines
Ensure default network access rule for Storage Accounts is set to deny	High	main.tf	VIEW
Ensure storage for critical data are encrypted with Customer Managed Key	High	main.tf	VIEW
Ensure default network access rule for Storage Accounts is set to deny	High	main.tf	VIEW
Ensure default network access rule for Storage Accounts is set to deny	High	main.tf	VIEW
Ensure that Storage Accounts use customer-managed key for encryption	Medium	main.tf	VIEW

[ipcrm]

Awesome!

[lacework-code-security]

(Audit Mode) Lacework IAC found 5 new violations

Violation	Severity	File	Guidelines
Ensure storage for critical data are encrypted with Customer Managed Key	High	main.tf	VIEW
Ensure default network access rule for Storage Accounts is set to deny	High	main.tf	VIEW
Ensure default network access rule for Storage Accounts is set to deny	High	main.tf	VIEW
Ensure default network access rule for Storage Accounts is set to deny	High	main.tf	VIEW
Ensure that Storage Accounts use customer-managed key for encryption	Medium	main.tf	VIEW