Make porter home portable

[carolynvs]

Right now porter's home is glued to your local file system which isn't conducive to collaboration. Instead the data stored in porter's home directory: claims, credential sets should be storable anywhere: in a database, blob storage, somewhere in a cloud.

1. Wire up porter to use plugins to R/W data from porter home instead of assuming the file system
2. Write an internal plugin to implement the current functionality and default to it, preserving current behavior.
3. Make porter configurable so that people can pick which plugin to use.

[carolynvs]

Closed by #859