Add “default” storage used by all DSR policies by default

[mfbrown]

Today, every DSR policy rule needs to explicitly link to a named “storage destination” that it uses for uploads. This allows the use of different storage destinations for different DSR policies, but in practice the more likely use case is that a customer would want to use the same storage configuration for all their policies.

Ultimately with the eventual UI, the steps for setting up and using a storage location will look like this:

• Add “default” storage used by all DSR policies by default - this should be explicitly stated on the storage location config itself.
• Admin UI to configure “default” storage
• Settings variables to configure “default” storage

This issue will satisfy the final bullet, in the above list, but we can do this work before the UIs are complete.

AC

• I can specify the default storage location in the config
• Update the policies to use the default when nothing else is configured.

[adamsachs]

implementation note based on offline conversations --

we should implement the default storage config on the backend in such a way that would allow users to keep multiple storage configs on the system simultaneously, with only one at any time being selected as the active default storage configuration