Refactor network service to separate out security groups

[richardcase]

/kind feature

Describe the solution you'd like
For the EKS implementation we need to be able to reconcile the network without the security groups. The security groups will be handled differently with the managed side of the provider

Anything else you would like to add:
Needed for #1787

Environment:

• Cluster-api-provider-aws version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[richardcase]

/assign
/lifecycle active