Feature/provisioner multitenant

first version of the multitenant provisioner master, required for #212 .  a couple high-level items:
* Docs will be added (working on those now)
* See the diff for some minor changes to the existing provisioner (now: worker)
* For simplicity, added this under provisioner/multitenant folder, and the ruby calls up to '../daemon/provisioner' for the worker.  There will be a followup PR to restructure the provisioner directory.  
* try it out, switch to the branch and from ``provisioner/multitenant`` run:
   * ``./bin/provisioner --uri http://localhost:55054`` (-h for other options, default binding is 0.0.0.0:55056)
   * add workers: ``./spec/tenant_worker_request.rb mytenant 3``
   * delete tenant: ``curl -v -X DELETE http://localhost:55056/v1/tenants/mytenant``
   * get heartbeat stats: ``curl http://localhost:55056/heartbeat``
* basic rspec testing, run rake
* rubocop is happy (2 minor offenses which will be addressed when we iron out the plugin data stuff)



this class is only used internally, where the tenant/provisioner/name instance variables are explicitly set by the tenantmanager, and from values either generated internally (provisioner_id, name) or from the request (tenant_id).  The main purpose is just so that it can be stubbed/replaced for testing, so i dont think this is the right place for validation