Path selection should prefer path with shorter cluster list length

[Nischal Sheth]

Best path election logic should prefer path with shorter cluster list
length. A path with no cluster list if considered to have 0 length.

This comes into play when CNs are RR clients and also have direct
iBGP peering. Directly learnt paths should be preferred over paths
learnt from RR.

See step 14 of JUNOS path selection algorithm:

https://www.juniper.net/documentation/en_US/junos13.1/topics/reference/general/routing-ptotocols-address-representation.html

[OpenContrail Admin (ci-admin-f)]

Review in progress for https://review.opencontrail.org/16193
Submitter: Nischal Sheth (<email address hidden>)

[OpenContrail Admin (ci-admin-f)]

Reviewed: https://review.opencontrail.org/16193
Committed: http://github.org/Juniper/contrail-controller/commit/f4cfa0b2a9a36f5bec917b7306aed4dd4d17f9ea
Submitter: Zuul
Branch: master

commit f4cfa0b2a9a36f5bec917b7306aed4dd4d17f9ea
Author: Nischal Sheth <email address hidden>
Date: Wed Jan 6 17:30:08 2016 -0800

Prefer path with shorter cluster list length

Highlights:

- Define ClusterListSpec, ClusterList and ClusterListDB
- Use cluster list length in path selection
- Add cluster list to route introspect
- Add unit tests for decode/encode and path selection

Change-Id: If7c15cc84d85fe31f8aafc48b54586457806e433
Closes-Bug: 1531679