Further EVPN changes for JUNOS interop.

Add ethernet-tag field to EnetAddressType in the schema.
Encode and decode ethernet-tag in XMPP messages.
Tweak provision_bgp.py to enable e-vpn on all BGP sessions.
Merge pull request #16 from Juniper/nsheth-evpn-devel

Nsheth evpn devel