Reduce the length of GRE interface name

Summary:
The define IFNAMSIZ in Linux kernel defines the maximum length of an interface
name. By default, 16 characters, including the final '\0'.

As such, names like gre-drake_via_router-001 are too long.

gre-drake seems to be descriptive enough, as a node has only one GRE tunnel
if it's not our router.

Reference: include/uapi/linux/if.h

Ref T1832

Test Plan: Deploy on cloudhugger, ifconfig gre-drake

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1832

Differential Revision: https://devcentral.nasqueron.org/D3005