Improve UT coverage of tacker

Currently, UT coverage of tacker is 68% in average, some new features like kubernets-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 tacker's key feature's UT coverage up close to 100%. Since tacker has large amout of code, in this blueprint, it is better to complete improvement by feature in step. The assuming plan is as follows:

High priority:
* Tacker server
* DB objects
* Tacker-conductor
* Vnflcm-driver
* Kubernetes-driver

Medium priorty:
* Legacy tacker-server
* NFVO
* Openstack-driver
* Vnfm
* infradriver
* DB connections

Low priorty:
* Monitor-driver
* common-utils
* Policy-actions
* Mgmt-driver
* Others