Fixes unit-tests in test_dbaas.py
Fixes unit-tests in test_dbaas.py

- Updated test_mongodb_error_in_write_config_verify_unlink,
  to mock operating_system.move instead of mocking execute_with_timeout.

- test_mongodb_error_in_write_config and
  test_mongodb_error_in_write_config_verify_unlink are nearly same,
  so removed test_mongodb_error_in_write_config.

Change-Id: I391dda4696ba30f24b2f9efd68a7d617010d680c
Closes-Bug: #1446105

mmasaki/trove,hplustree/trove,zhangg/trove,zhangg/trove,redhat-openstack/trove,mmasaki/trove,openstack/trove,redhat-openstack/trove,fabian4/trove,cp16net/trove,openstack/trove,mmasaki/trove,fabian4/trove,redhat-openstack/trove,cp16net/trove,hplustree/trove,fabian4/trove,cp16net/trove