core: Config mocking in StoragePoolValidatorTest

Used the standard was of mocking Config - MockConfigRule.

This refactoring enables removing the useless getConfigValue method
from the StoragePoolValidator class, and just call Config.GetValue
directly instead.

Change-Id: I03c0f2a1c013cd12010be7f84d896c1d1e600c29
Signed-off-by: Allon Mureinik <amureini@redhat.com>