Fix IPv6 provisionning

Summary:
Eglide didn't automatically get back IPv6 connectivity
when reprovisionned lastly.

After investigation, two issues was found in the tunnel initialisation
script: it wasn't marked as executable, as Salt created by default
files with chmod 644, and variables initialisation was prefixed by $.

This is a follow-up for 71ac80215b1f.

Ref T768. Fixes T1059.

Test Plan: Run on Eglide

Reviewers: Sandlayth, dereckson

Reviewed By: dereckson

Subscribers: amj

Maniphest Tasks: T1059, T768

Differential Revision: https://devcentral.nasqueron.org/D749