testutil: Should ensure `consul` executable is correct version or use embedded code

Client: `0.7.0`
Server: `0.7.0`

### Description of the Issue (and unexpected/desired result)

Products such as `consul-template` use `testutil` to spin up Consul servers. While developing a patch I was hitting unexpected issues in tests. The cause was a local `consul` binary being out of date. Note that `consul-template` includes the full `consul` repo as a vendor source.

### Reproduction steps

* Install `consul` binary 0.6.3 into the path
* Download `consul-template` and issue `make test`

Various failures are experienced.

Upgrade `consul` binary to 0.7.0 and issues are resolved.

### Suggestion

It would be nice if the test util did one of the following:

* validated the version of the Consul binary it uses to match the configured version
* validated the version of the Consul binary it uses to a supported version
* used the local Consul code to spin up the server
* At least indicated the mismatch