Use values from DB instead of config file for tempest config.

Update tempest_tester.py to use the data in the cloud db.
Values in config file can still be used to futher customize the tempest config.
Unnecessary values are removed from conf.json.sample.
The remaining values will be removed once the execute_test.py can discover
them.

Change-Id: I5a3257a0b99985fccbc0f26d04f0060986ec6cc3
Merge "Use values from DB instead of config file for tempest config."