Allow origins to export multiple prefixes

[jhiemstrawisc]

This should be the first step toward multi-export origins. While I didn't get
things sorted out for S3, the fact is the S3 plugin doesn't really support that
yet anyway. For now, this appears to be working with POSIX, which is the majority
of origins anyway.

In particular, the larges addition as I see it is the GetOriginExports function,
which allows us to construct a list of OriginExports that can be used in a standard
way elsewhere.

Closees #858

[jhiemstrawisc]

@CannonLock Just wanted to ping you to make sure you see the new addition here from over the weekend. We now have the object:

Origin:
  Exports:
    - StoragePrefix: "/foo"
       FederationPrefix: "/bar"
       Capabilities: ["PublicReads", "DirectReads", "Writes", "Listings"]
    - <Another export>

[turetske]

Preliminary review - still need to dig deep into some of the code.

[turetske]

Other files that also contain environment params to fix: github_scripts/stat_test.sh

[CannonLock]

@jhiemstrawisc Is this the full set of values that Capabilities can be?

Capabilities: ["PublicReads", "DirectReads", "Writes", "Listings"]

[jhiemstrawisc]

@jhiemstrawisc Is this the full set of values that Capabilities can be?

Capabilities: ["PublicReads", "DirectReads", "Writes", "Listings"]

Good catch, I missed one there. There's also "Reads", which turns on authenticated reads. Setting "PublicReads" implies "Reads" (you can't read publicly if you don't allow any reading at all), but a capabilities flow without either "Reads" or "PublicReads" implies that it's a write-only origin.

[CannonLock]

@jhiemstrawisc can multiple values be true at the same time or can you only pick one?

[turetske]

@jhiemstrawisc can multiple values be true at the same time or can you only pick one?

Multiple values. You can have Writes and PublicReads both be true, for example.

[turetske]

This seems good to me, but obviously needs a rebase for the merge conflicts and hopefully Cannon's fixes in there, soon.

Trying to force tests again