Add basic support for a few EVPN communities

Decode and display the Default Gateway, ES Import and ESI Label
extended communities.  More elaborate support can be added as
needed when the implementation needs to add these communities
to evpn routes.

Change-Id: I3ff61b48c47c37d77f9a4065867873f4cd5f8ce0
Closes-Bug: 1386453
Merge "Add basic support for a few EVPN communities"