bgp: T4490: Add informational message for peer withour AFI

[sever-sever]

Change Summary

As we don't use address-family ipv4-unicast by default we
should sending an informational message about AFI for the peer is required
Cosmetic warning

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://phabricator.vyos.net/T4490

Component(s) name

bgp

Proposed changes

How to test

set protocols bgp local-as 65001
set protocols bgp neighbor 10.0.0.254 remote-as 65001

Commit:

vyos@r14# commit
[ protocols bgp ]
Neighbor "10.0.0.254" requires address-family!

[edit]
vyos@r14# compare 
No changes between working and active configurations.
[edit]
vyos@r14# 

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly