Backwards Incompatible Changes in Nomad 1.7 Driver RPC API

[jseba]

There was a change to the Nomad Driver config sent by SetConfig in 1.7 that breaks running drivers built with the 1.7 APIs against a Nomad cluster running on previous versions. There is an API version field in the base config struct sent by Nomad and it was not incremented with this breaking change. This makes it impossible for the driver to decide if it should attempt to parse the Topology field in ClientDriverConfig.

The driver RPC version should have been incremented here so that drivers could determine what version of the struct they are getting from Nomad.