add TIRAMISU_STORAGE environment variable

ChangeLog

@@ -1,3 +1,8 @@
+Fri Jun 20 16:27:55 2014 +0200 Emmanuel Garette <egarette@cadoles.com>
+
+	* add TIRAMISU_STORAGE for personalise default storage in
+	environnement
+
 Thu Jun 19 23:20:29 2014 +0200 Emmanuel Garette <egarette@cadoles.com>
 
 	* add DynOptionDescription:

tiramisu/storage/__init__.py

@@ -1,4 +1,4 @@
-# Copyright (C) 2013 Team tiramisu (see AUTHORS for all contributors)
+# Copyright (C) 2013-2014 Team tiramisu (see AUTHORS for all contributors)
 #
 # This program is free software: you can redistribute it and/or modify it
 # under the terms of the GNU Lesser General Public License as published by the
@@ -27,6 +27,7 @@ configurator ``set_storage()``.
 
 
 from time import time
+import os
 from tiramisu.error import ConfigError
 from tiramisu.i18n import _
 
@@ -36,7 +37,7 @@ class StorageType(object):
     default storage is store as selected storage. You cannot change it
     after.
     """
-    default_storage = 'dictionary'
+    default_storage = os.environ.get('TIRAMISU_STORAGE', 'dictionary')
     storage_type = None
     mod = None
 
@@ -52,7 +53,11 @@ class StorageType(object):
             self.storage_type = self.default_storage
         if self.mod is None:
             modulepath = 'tiramisu.storage.{0}'.format(self.storage_type)
-            mod = __import__(modulepath)
+            try:
+                mod = __import__(modulepath)
+            except ImportError:
+                raise SystemError(_('cannot import the storage {0}').format(
+                    self.default_storage))
             for token in modulepath.split(".")[1:]:
                 mod = getattr(mod, token)
             self.mod = mod