Reconfigure GRE tunnels

Summary:
Simplify the GRE tunnels configuration. This change allow to define tunnels
in one block for both endpoints.

Recreate tunnels against router-001, instead of one-one connections.

Test Plan: salt * node.resolve_gre_tunnels

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2594