Fix startup with half-populated db

Fix startup with half-populated db

In some cases, Cynara could start with half-populated database.
As this is potential security issue, we now make sure, that in case
of any error, Cynara will start with empty database and return DENY
for every request.

There are added tests revealing these potential issues.
Some test cases depend on specific state of Cynara's policy database
directory. These are now provided in cynara-tests package and placed
in /usr/share/cynara/tests/ during installation.
Test execution does not affect real database -- it uses above tests
path only, accessible by non-root users as well.

Signed-off-by: Aleksander Zdyb <a.zdyb@partner.samsung.com>
Signed-off-by: Pawel Wieczorek <p.wieczorek2@samsung.com>
Change-Id: Ia943f77a2a0c85f394c40dd10333a73df4d0c96a