Pass plugin config via stdin

[carolynvs]

What does this change

Reads optional configuration from the porter configuration file and passes it to the plugin on stdin as json.

It is not sent as part of the plugin interface/contract because each plugin may or may not have config, it may look completely different or be loaded from other places, and I'd like to have the plugin contract mimic the cnab contracts. For example the interface for claims is the CrudStore.

default-instance-store = "porterci"

[[instance-store]]
  name = "porterci"
  plugin = "azure.blob"

  [instance-store.config]
    env = "PORTERCI_AZURE_STORAGE_CONNECTION_STRING"

What issue does it fix

N/A

Notes for the reviewer

This relies on a patch: hashicorp/go-plugin#127

This is the first step in getting a general framework in place for plugins. Next steps after this will be to make the pluginProvider generic so it can load a plugin and config for any interface. Right now I'm just feeling out what works.

[carolynvs]

/azp run porter
/azp run porter-integration

[azure-pipelines]

No pipelines are associated with this pull request.

[vdice]

LGTM.

Should we add some sample porter configs to the docs? Or hold off, perhaps if more updates are planned?

[carolynvs]

I was going to hold off until someone can install a plugin without having to compile it themselves.