Change behavior of trust-anchor

- Trust-anchor certificate file directory has fixed.
- Remove pkg-certs-dir attribute due to change of its concept.

Related changes:
[app-installers] : https://review.tizen.org/gerrit/149836
[wgt-backend] : https://review.tizen.org/gerrit/149978
[tpk-manifest-handlers] : https://review.tizen.org/gerrit/150060
[wgt-manifest-handlers] : https://review.tizen.org/gerrit/150136

Change-Id: I746853898e3206ccc00cd543d3c7ca90c473feca
Signed-off-by: Junghyun Yeon <jungh.yeon@samsung.com>