Preserve SiteOfOrigin when creating ServiceChain routes

Following changes are implemented:

- Infra for SiteOfOrigin (aka RouteOrigin) community
- ServiceChain changes to preserve SiteOfOrigin from original route

Change-Id: I9b26ed7ef148a5e367db5deb226bcaeafad44db3
Partial-Bug: 1383721
Add basic support for some EVPN communities

Change-Id: I3ff61b48c47c37d77f9a4065867873f4cd5f8ce0
Add basic support for some EVPN communities

Change-Id: If66400a3495775315f4ef37a0c2e1c3e5e7a4ea3