Added check to make sure default interface is always first in list.

Added check to make sure default interface is always first in list.

This changeset ensures the default interface, if present, is always
first in the interface list. This addresses spec compliance for section
7.1.4.2.3.
Also added unit tests to test changes made in this changeset and to test
for duplicate interfaces.

Change-Id: Iccfbb297c1745a86ce2cb0010f5b24519d567658
Signed-off-by: Mandeep Shetty <mandeep.shetty@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/954
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Erich Keane <erich.keane@intel.com>