Reduce use of powermockito

Reduce use of powermockito

Rather than mucking with powermock, use Mockito's partial mocking
on a real instance of HwvtepConnectionManager.

Also proper application of @VisibleForTesting and ovsdbClient mocking
allows us to not mock HwvtepConnectionInstance at all.

We also have a simple utility for setting final fields, use that
to reset Operations static field -- allowing us to not use powermock
in DataChangeListenerTestBase at all.

JIRA: OVSDB-476
Change-Id: I6c321c0455fb886dc0029b2a321f73eb3ba150d1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>