Server and client may not read baseline GCD from the same path

[mlincett]

In the current design, server and client are expected to read the baseline GCD from the same path, but this is not guaranteed to happen.

In #250 I tentatively implement the data staging for the baseline GCD, so that the server can read from container / HTTP and the client can read from container / CVMFS. Note that more cleanup is needed and that branch will likely be merged to #244 .

As @dsschult mentioned, it is also possible to give CVFMS access to the server (this is more of a SkyDriver issue).

I believe the "startup_json" now contains the baseline GCD path, but once the data stager is in place only the filename is required.