Make backup-related configuration entries optional.

Complete list:
backup-script, full-backup-directory, incremental-backup-directory,
innobackupex-incremental, innobackupex-full, innobackupex-binary,
xtrabackup-binary

slapos/recipe/generic_mysql/__init__.py

@@ -127,6 +127,8 @@ class Recipe(GenericBaseRecipe):
        )]
     )
     path_list.append(mysqld)
+    # TODO: move to a separate recipe (ack'ed by Cedric)
+    if 'backup-script' in self.options:
       # backup configuration
       full_backup = self.options['full-backup-directory']
       incremental_backup = self.options['incremental-backup-directory']