Restrict project-tree depth

Controlled by keystone.conf on the 'max_project_tree_depth' item.
Configuration on keystone.conf (default on keystone/common/config.py - DONE
Check depth on creating new project (SQL and KVS) - DONE
Unit tests for creating new project (SQL and KVS) - DONE
Check depth on changing parent of leaf project - DONE
Unit tests for changing parent of leaf project - DONE