Add method to get privilege level from file

Add method to get privilege level from file

- Signature1 of package has been erased after installation in earlier tizen.
- But signature is needed during migration to latest platform version
  to determine privilege level and to be stored at cert db for trusted operations.
- So, add some codes to get dist root cert value from file which is made by
  migration script.
- But backend still need to assume privilege level for preload RPM packages.

Change-Id: Idf2fa96fb6c4fbf3e598df48e7d93bf367228628
Signed-off-by: Junghyun Yeon <jungh.yeon@samsung.com>