Use consistent format for families in BgpPeerInfoData.

Add negotiated_families field to BgpPeerInfoData.
Use same format as the configuration e.g. inet-vpn for
configured_families, families and negotiated_families.
Fix types for Afi and Safi.
Merge pull request #504 from Juniper/nsheth_bug_2705

Use consistent format for families in BgpPeerInfoData.