(POOLER-160) Add Prometheus to pooler startup

This is a re-architect of the vmpooler initialisation code to:
1. Allow an API service for both manager and the api
2. Add the Prometheus endpoints to the web service.
   Needed to change the way the Rack Service is started as instantiating
   using ".New" leads to a failure to initialise the http Stats
   collection.
3. Selectively load the pooler api and/or Prometheus endpoints.
4. Rework API Spec tests for revised API loading. Needed to tidy up the
   initialisation and perform a reset! after each test to avoid "leaks"
   and dependencies between the tests.

spec/integration/api/v1/vm_template_spec.rb

@@ -8,6 +8,13 @@ describe Vmpooler::API::V1 do
     Vmpooler::API
   end
 
+  # Added to ensure no leakage in rack state from previous tests.
+  # Removes all routes, filters, middleware and extension hooks from the current class
+  # https://rubydoc.info/gems/sinatra/Sinatra/Base#reset!-class_method 
+  before(:each) do
+    app.reset!
+  end
+
   describe '/vm/:template' do
     let(:prefix) { '/api/v1' }
     let(:metrics) { Vmpooler::DummyStatsd.new }
@@ -33,9 +40,8 @@ describe Vmpooler::API::V1 do
     let(:checkoutlock) { Mutex.new }
 
     before(:each) do
-      app.settings.set :config, config
-      app.settings.set :redis, redis
-      app.settings.set :metrics, metrics
+      expect(app).to receive(:run!).once
+      app.execute(['api'], config, redis, metrics)
       app.settings.set :config, auth: false
       app.settings.set :checkoutlock, checkoutlock
       create_token('abcdefghijklmnopqrstuvwxyz012345', 'jdoe', current_time)
@@ -84,8 +90,8 @@ describe Vmpooler::API::V1 do
       end
 
       it 'returns 503 for empty pool when aliases are not defined' do
-        Vmpooler::API.settings.config.delete(:alias)
-        Vmpooler::API.settings.config[:pool_names] = ['pool1', 'pool2']
+        app.settings.config.delete(:alias)
+        app.settings.config[:pool_names] = ['pool1', 'pool2']
 
         create_ready_vm 'pool1', 'abcdefghijklmnop', redis
         post "#{prefix}/vm/pool1"