T57858: mountvofs anonymously

I added an integration test for anonymous mounts, and then changed some of the logic in the Client constructor to handle defaults for certfile differently (it seemed to ignore the mountvofs command line and always take ~/.ssl/cadcproxy.pem).

@ijiraq Yes, I wasn't sure which should take precedence. It's easy enough to switch around the logic. Actually, to avoid ambiguity, perhaps it should raise an exception if both conn and certfile are specified (and they disagree?). Or only take conn as an argument to the Client constructor (which would require fiddling all of the vcp, vrm etc. commands). Let me know which solution you prefer.