Add support for provisioning a cluster in an existing VCN (i.e. shared VCN)

[jlamillan]

Currently, each cluster creates a brand new VCN, subnets, reoute tables, etc. for each cluster. It would be nice if we supported the ability to provision subsequent clusters into an an existing VCN.

[jlamillan]

See also #77

[garthy]

This is also needed #168

[beide]

I want to use existing VCN features. is the enhancement ready for me to use? Thanks

[AdityaVishwekar]

Workaround -
Inside 'network/vcn' folder :

1. Comment out resource "oci_core_virtual_network"
2. Replace "oci_core_virtual_network.CompleteVCN.id" with "var.vz_vcn_id" and "oci_core_virtual_network.CompleteVCN.default_dhcp_options_id" with "var.vz_vcn_dhcpoptions_id" in securitylists.tf, subnets.tf, vcn.tf
3. Define variables in variables.tf -
   `variable "vz_vcn_id" {
   default = "Put vcn ocid"
   }

variable "vz_vcn_dhcpoptions_id" {
default = "Put dhcpoption ocid"
} `