Support uri origin parameter

This pull request is for our staging environment testing.

I need to change the default uri origin(https://mackerel.io) because our staging environment's origin is different from https://mackerel.io.

mackerel.io client in ruby has mackerel_origin parameter, so it's easy to support configuring uri origin.  
https://github.com/mackerelio/mackerel-client-ruby/blob/master/lib/mackerel/client.rb#L12