Allow creating vpc flow logs resources without creating vpc

[csautter]

Is your request related to a problem? Please describe.

I have a VPC network which is created outside of this module. But I like to use this module for creating resources like vpc flow logs in the existing VPC network. Setting the variable create_vpc=false let me assume that i can disable the creation of the vpc itself but can use the other module functions.

Describe the solution you'd like.

If I set an existing vpc_id it should be possible to create resources like vpc flow logs without creating a VPC.

Additional context

I created a working solution for vpc flow logs: https://github.com/csautter/terraform-aws-vpc/tree/dev/optional-vpc-id-var and have tested it with following vars:

  create_vpc                           = true
  vpc_id                               = "id"
  enable_flow_log                      = true
  create_flow_log_cloudwatch_log_group = true
  create_flow_log_cloudwatch_iam_role  = true

I have some time to work out a contribution ready solution.