Read the CARP peer parameter from pillar

Summary:
Parameter is optional.

Ref T2274

Test Plan: Tested the function (get_carp_entries) with CARP entries containing a peer

Reviewers: dereckson, Duranzed

Reviewed By: dereckson, Duranzed

Subscribers: Duranzed, dereckson

Maniphest Tasks: T2274

Differential Revision: https://devcentral.nasqueron.org/D4009