BUG-5006: rework SouthboundProviderTest, clean up

SouthboundProviderTest doesn't need PowerMock, rework it to drive
everything through public methods and verify data-store side-effects
instead of method invocations.

Clean up a few unneeded exception declarations on close() methods.

Remove the last traces of PowerMock from SouthboundMapperTest.

Change-Id: I9b634680211fa528780b6a8b9edc4153744aba08
Signed-off-by: Stephen Kitt <skitt@redhat.com>