Move tests db.api.instance_* to own class

In this change tests for instance_* moved to own class.

blueprint db-api-tests

Change-Id: Id0a2e4a6736cce9a1a3681ba8617cf0b40dd9678
Merge "Move tests db.api.instance_* to own class"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp