Enable graceful upgrading/modifying of configured repos within Nexus shared service

[jjgriff93]

Is your feature request related to a problem? Please describe.
Nexus currently doesn't implement upgrade (and shared services more widely as of writing), meaning the only way to currently configure new proxy repos etc. is to do so directly within the Nexus admin interface. The issue with this is if the service is redeployed at any point, these configurations will disappear and it will revert to the json configurations within the bundle.

The proper path to adding/modifying/removing repo configurations would be to amend them in the bundle so they are persistent; however this would then mean the current Nexus Vm would need to be destroyed and re-deployed for these to be copied over and applied.

Describe the solution you'd like
We need a method to allow modified/new json configurations to be copied to the existing VM and then applied via the Nexus API using a script. This only happens on create right now as part of cloud-init so this could be split out and ran from within the porter actions instead; or alternatively we need a way to get terraform to be aware of configuration file changes.

Describe alternatives you've considered
Using a file array within terraform cloud-init block where we manually list the configuration files (instead of just copying a folder using the file provisioner); however this won't respect modified files.