Bug 3332: Fix connections made to an ODL instance

If an OVSDB connection is made via OVSDB SB MD-SAL
to self (same ODL) or another ODL instacne, the
OVSDB SB provider would hang or otherwise stop working.
This fixes by returning a null list to the 'list_dbs'
command.

v2 - remove white space
     When an OVSDB connection is established,
     the first request that the OVSDB Southbound makes is a
     list_dbs command.  By returning an empty db list, two
     things occur:
     1 - the request gets a response
     2 - the empty db list signals an uninteresting connection

v3 - rebase for stable/lithium

Change-Id: I295dd47b821f7ec4ead0eb489a1b124cc574e8cf
Signed-off-by: Eric Multanen <eric.w.multanen@intel.com>