Investigate configuration strategy

[carolynvs]

Right now we have multiple ways to configure the plugin:

• Use the default storage connection string env var for blob storage
• Specify an alt storage connection string env var
• Use the default service principal env vars for key vault
• Specify a prefix to apply to the the service principal env vars

And looming in the wings is being able to log in with az login (MSI) which I have partway implemented.

Investigate if we can bring this down a bit so that there aren't so many ways to do this, it's confusing to have one way to auth for storage and another for everything else.

See #6 (comment), as part of this let's try to cleanup the config struct if we can as well.