Bug 2192 - Fix access to custom.properties, so non-default values can be provided

When OVSDB changed to use Karaf, the custom variables in config.ini got moved to
custom.properties. With that change, OVSDB was not grabbing its custom values from
the right place, causing all OVSDB related properties to get ignored.

Change-Id: Ic43d547e3d30e69c86718ac03314c4b74c61b561
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>