Make controller role checking more reliable

Make controller role checking more reliable

When there are two active OpenFlow channel connections configured,
the role of each connection keeps changing as the two connections
compete and keep reconnecting. So checking the controller role
every second is not a very reliable way to detect transient state.

Use OVSDB monitoring which streams updates in the database to
record all the changes of the controller role, instead of periodic
polling, to make the test results more reliable. This will help
to detect true failure cases.

Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Change-Id: Ided77a53e4df7db1ef99d453cfd7aa0dcd062532