Read config file path from TRYTOND_CONFIG env variable if no config file is specified.

issue3323
review1004002
Improve MPTT

- Remove the order on MPTT
- Make room with queries using left/right instead of ids
- Call _update_tree multiple times for small list instead of rebuild the all
  tree
- Improve test for with many roots
- Factorize the test to update MPTT

issue1892
review1007002
merge