Special values for <serverHostname>: IP and HOSTNAME

Currently, `<serverHostname>` is hard-coded, i.e. it has to be specified on a per-machine basis. To address this, I propose these two "special values":

* `IP`: the current (local) IP.

 Caveat: in the case of hosts with "complicated networking" (i.e. multi-homed), this might give you the "wrong" IP (i.e. not the one you wanted).

* `HOSTNAME`: the current (local) hostname.

 Caveat: must be resolvable by the remote selenium server (i.e. it won't work if the local hostname in not on DNS).