Fixes #2478 #2474 #1995 #2294

The new client only handles agent and task service advertisement. Server
discovery is mostly unchanged.

The Nomad client agent now handles all Consul operations instead of the
executor handling task related operations. When upgrading from an
earlier version of Nomad existing executors will be told to deregister
from Consul so that the Nomad agent can re-register the task's services
and checks.

Drivers - other than qemu - now support an Exec method for executing
abritrary commands in a task's environment. This is used to implement
script checks.

Interfaces are used extensively to avoid interacting with Consul in
tests that don't assert any Consul related behavior.