(maint) Enable Ctrl-C to kill all threads in developer environment

Previously, if you ran the vpooler via ruby, pressing Ctrl-C would terminate the
Webserver however the PoolManager does not have a handler and would instead
just keep executing.  This commit adds a global Ctrl-C hook which terminates
both the api and manager threads.  This behaviour will only be enabled if the
`VMPOOLER_DEBUG` environment variable exists so that it does not affect VMPooler
when running in production environments.

vmpooler

@@ -26,4 +26,11 @@ manager = Thread.new {
   ).execute!
 }
 
+if ENV['VMPOOLER_DEBUG']
+  trap("INT") {
+    puts "Shutting down."
+    [api, manager].each { |t| t.exit }
+  }
+end
+
 [api, manager].each { |t| t.join }