the function of create_test_server_group should be used instead
create_test_aggregate
Change-Id: I383bc563d369144a5f918064309b73e7984b61ab
Closes-Bug: #1736331
This change add ServerGroup, ServerGroupPolicy and ServerGroupMember
models, and add several basic DB interfaces.
Partially Implements: bp server-group-api-extension
Change-Id: I86e17798718fc26e1fd6a1fe1c33ee408738b6da