Remove limitations in transit VN functionality

[Nischal Sheth]

Transit VN functionality was implemented via bug 1365277.

https://bugs.launchpad.net/juniperopenstack/+bug/1365277

However, out of the 4 cases listed in that bug, only 1 case is
currently supported. Specifically, cases only case c) is supported.

a) VN1---VNx---VN2
b) VN1---SC1x---VNx---VN2
c) VN1---SC1x---VNx---SC2x---VN2
d) VN1---VNx---SC2x---VN2

Need to also support the other 3 cases. Further it would
be useful to support more arbitrary graph of transit and
non-transit VNs. For example (VNx and VNy are transit VNs)

VN1---VNx---VNy---VN2
VN1---VNx---VNy---SC2y---VN2
VN1---VNx---VN2---VNy---VN3
VN1---VNx---VN2---VNy---SC3y---VN3

The loop prevention mechanism implemented as part of the fix
for bug 1401010 should allow more arbitrary topologies of VNs
without the possibility of creating loops.

https://bugs.launchpad.net/juniperopenstack/+bug/1401010