add support for multiple compartments as per sod

[srirg]

This PR is a fix for #166

[garthy]

I'm not sure the multiple_compartments flag is needed. Can you just use the value if set? e.g:-

network_compartment_ocid = "${(var.network_compartment_ocid != "") ? var.network_compartment_ocid : var.compartment_ocid}"

Also can you rebase + squash to master and split the functionally out into a clean merge request with structured descriptive commit message(s). At the moment this change has many features including a bastion change with many unneeded commit messages.

Thanks