jobspec: add service block provider parameter

[jrasell]

This PR adds a new parameter to the service block named provider. This dictates which service registration provider to use and defaults to consul maintaining the current implementation default. The parameter can also be nomad; any other value will result in an error being returned to the user upon job registration or similar.

The validation of services now includes specific validations depending on the provider configuration parameter. The MVP also only allows a single provider to be used per task group.

closes https://github.com/hashicorp/team-nomad/issues/264

[schmichael]

I think you'll also need to check helper methods like Job.ConsulUsages and make sure they only check services using the Consul provider.

There's probably a lot more places that assume len(services) > 0 means Consul is going to be used, so I'll try to keep exploring as you land more PRs.

[hashicorp-cla]

All committers have signed the CLA.

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