Added dtrust module support cross-domain token

Blueprint domain-trusts

Partly modified trust module into dtrust module. Support of cross-domain token
issuing and validation. Domain-trust table added to mysql.

TODO:
1. more sophisticated token composition
2. full-fledged dtrust module
3. migration script for database

Change-Id: Ica82008f61552a63a7e7dbf5f4c9cc6993ea7b57
Signed-off-by: Bo Tang <btang@cs.utsa.edu>