Add support for default datacenter

[miklinux]

It would be really helpful if the default datacenter could be set at configuration time

# Initialize Consul
Diplomat.configure do |config|
   config.datacenter = 'mydc'
end

and all API calls would use that each time, unless overridden.

[pierresouchay]

@miklinux Yes, that's a good idea, would you like to propose a PR, I'll be glad to review it

[miklinux]

@pierresouchay Will look into it

[miklinux]

Hi @pierresouchay,
here's the PR for reviewing!

#220