core: Make MockConfigRule work with Versions

Fixed MockConfigRule to work with the Version class so it provides a
meaningful interface instead of just passing any String which is just
implementation detail in the Config class.

This patch contains the following:
* Fixing MockConfigRule's interface
* Adjusting AddVmCommandTest, GetConfigurationValueQueryTest and
  GetDeviceListQueryTest to pass the Version they use instead of its
  text value.
* Fixing VmNicValidatorTest to send the Version used in the validator to
  MockConfigRule.

Change-Id: I9f407bc1af73e76e223a8cbb1c67009f7dd97d61
Signed-off-by: Allon Mureinik <amureini@redhat.com>