improve ut coverage of DB object related files

Currently, UT coverage of tacker is 68% in average, some new features like kubernetes-driver, new db objects, and some core features like tacker server and tacker conductor is still not full covered by UT yet.

This blueprint is aim to improve DB objects related files' UT coverage over 90%. Since tacker has large amout of code, in this blueprint, it is better to complete improvement by feature in step. The blueprint will include these files:

* DB objects(current coverage)
  tacker/objects/vnf_deployment_flavour.py 88%
  tacker/objects/vnf_instance.py 74%
  tacker/objects/vnf_instantiated_info.py 82%
  tacker/objects/vnf_package.py 87%
  tacker/objects/vnf_package_vnfd.py 54%
  tacker/objects/base.py 55%
  tacker/objects/grant_request.py 56%
  tacker/objects/heal_vnf_request.py 73%
  tacker/objects/vnf.py 69%
  tacker/objects/vnf_lcm_op_occs.py 56%
  tacker/objects/vnf_lcm_subscriptions.py 37%
  tacker/objects/vnfd.py 63%
  tacker/objects/vnfd_attribute.py 42%

Target :
  * over 90% test coverage
     * codes not called/not in functions will not in UT code, but are calculated as not coverd.
        Thus some file cannot reach 90% coverage.
  * cover all non-internal methods
  * merge in Wallaby