Update agent CLI to include Consul config option flags.

[jrasell]

This update introduces command line flags for all Consul options thus allowing users to both use a config file and CLI options to configure Consul.

Website documentation has also been updated.

The change was tested running a local build with the command nomad agent -dev -consul-server-service-name=jrasell-nomad -consul-client-service-name=jrasell-nomad-client. Consul confirming the correct configuration:

$ curl -s localhost:8500/v1/health/checks/jrasell-nomad | jq '.[0].ServiceName'
"jrasell-nomad"

 $ curl -s localhost:8500/v1/health/checks/jrasell-nomad-client | jq '.[0].ServiceName'
"jrasell-nomad-client"

Closes #3304

[dadgar]

@jrasell Awesome work!

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