Issue 3016 - Improve Nginx source => PPA migration steps

modules/nginx/manifests/init.pp

 class nginx (
     $worker_processes = $nginx::params::worker_processes,
     $worker_connections = $nginx::params::worker_connections,
     $ssl_session_cache =  $nginx::params::ssl_session_cache
   ) inherits nginx::params {
 
   apt::ppa {'ppa:nginx/stable':
   }
 
   apt::source {'nginx':
     ensure => 'absent',
   }
 
-  # Ensures that nginx is not installed from the Ubuntu sources
-  package {'nginx-common':
-    ensure => purged,
-    before => Package['nginx']
+  exec {'purge-nginx':
+    command => '/usr/bin/apt-get -y purge nginx',
+    logoutput => true,
+    path => '/usr/sbin:/usr/bin:/sbin:/bin',
+    refreshonly => true,
+    returns => [0, 100],
+    subscribe => Apt::Ppa['ppa:nginx/stable'],
   }
 
   package {'nginx':
     ensure => '1.8.0-1+precise1',
-    require => Apt::Ppa['ppa:nginx/stable'],
+    require => Exec['purge-nginx'],
   }
 
   File {
     owner => root,
     group => root,
     mode => 0644,
   }
 
   Exec {
     path => '/usr/bin:/bin',
@@ skipping 70 matching lines @@
           ensure => absent
         }
       }
     }
 
     if $enabled == true {
 
       if $is_default {
         $default_conf = '/etc/nginx/sites-enabled/default'
         ensure_resource('file', $default_conf, {ensure => 'absent'})
-        File[$default_conf] ~> Service['nginx']
+        Package['nginx'] -> File[$default_conf] ~> Service['nginx']
       }
 
       file {"/etc/nginx/sites-enabled/${domain}":
         ensure  => link,
         require => File["/etc/nginx/sites-available/${domain}"],
         target => "/etc/nginx/sites-available/${domain}",
         notify => Service['nginx']
       }
     }
 
@@ skipping 47 matching lines @@
   }
 
   service {'nginx':
     ensure => running,
     enable => true,
     restart => '/etc/init.d/nginx reload',
     hasstatus => true,
     require => File['/etc/nginx/nginx.conf']
   }
 }