Bug 1234: Force to use legacy OF plugin if it is installed.

This patch enables to prioritize AD-SAL protocol plugin services,
and gives higher priority to legacy OF plugin than sal-compatibility.

This patch never affects integration.git artifacts because legacy
OF plugin is filtered out if MD-SAL OF plugin is installed.

Change-Id: Iee40a6f724db2105f2ec36014a10872bad824e86
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>
Merge "Bug 1234: Force to use legacy OF plugin if it is installed."