With this change delete_flows will only remove flows matching the default
cookie of the bridge.
The uninstall_flows implementation in the native bridge is also modified
to touch only the flows with the bridge cookie.
To still allow deletion of all cookies, cookie=COOKIE_ANY is introduced
as a special value, and used in the agent code in the places where the
intent is indeed to clean all flows whatever their cookie is.
Partial-Bug: #1557620
Change-Id: Idd0531cedda87224531cb8fb6a912ccd0f1554d5