Save and use the last series_id and test_id for commands by default

It would be really nice to be able to do `pav run ...` and then `pav log` and similar commands without having to figure out what the last test id or series id was. 

We already do this with `pav cancel` and `pav status` based on the last series id. We should expand that to more commands. (With an error to standard out if there's more than one option for test_id from the last series).