Make sdnvpn logging proper

Make sdnvpn logging proper

* Currently no log messages are getting written into log file due to
file handler is registered for logger object created at different
modules. Now corresponding handler is registered for logger objects.
* Making sdnvpn Feature class to use its parent class logger object to
avoid unnecessary logger object creation.

Change-Id: I4ec61951ba4ac39cecc137dbb818da72f0b43b35
Signed-off-by: Periyasamy Palanisamy <periyasamy.palanisamy@ericsson.com>
(cherry picked from commit 303268d0464127c52f11443404fb1baae03d615f)