Alternative datastore for VidGrind testing

[Alan Noble]

While it is possible to deploy different instances of VidGrind for testing purposes, they currently all use the same datastore(s). It is proposed to have a completely separate datastore for testing, which would contain a full cross section of datastore entities (sites, devices, sensors, etc.). The test datastore would be configured by an environment variable that points to the required credentials (file or bucket).