Add check for left and right vn
Add check for left and right vn

left or right vn of a service chain could have been deleted while
schema transformer was down. When it comes back up, it tries to
read these networks. We should add a check for None before accessing
these networks to avoid generating a traceback.

Change-Id: If2d3ffccdc912eaa64a84e17233a2fb3e3a84477
Closes-Bug: 1590278
(cherry picked from commit d8518b11cc9d8a49451caa51b50c334c36189cfe)

nischalsheth/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,nischalsheth/contrail-controller,nischalsheth/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,eonpatapon/contrail-controller,nischalsheth/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,rombie/contrail-controller,eonpatapon/contrail-controller,codilime/contrail-controller,nischalsheth/contrail-controller,codilime/contrail-controller,codilime/contrail-controller,rombie/contrail-controller