Ensure pymysql is installed

PyMySQL was/will be removed from requirements.txt in refstack as it
is not an explicit dependency. It's an optional DB driver that is only
necessary if it is specified in the refstack configuration database URL.
Since pymysql is specified as the driver here, it is necessary to ensure
it is installed.

Change-Id: I336aa8ef196526601c16b165d8200b63ff11825d

manifests/app.pp

@@ -66,6 +66,13 @@ class refstack::app () {
     }
   }
 
+  if !defined(Package['pymysql']) {
+    package { 'pymysql':
+      ensure   => present,
+      provider => 'pip'
+    }
+  }
+
   # Create the RefStack configuration directory.
   file { '/etc/refstack':
     ensure => directory,