Check for IPv6 file before reading

Check to see if the IPv6 disabled flag file exists
before trying to read it. This file doesn't exist
on systems with the IPv6 module blacklisted.

Closes-Bug: #1361542
Change-Id: I4064b4189cc9d8f0f0e0e7025cf6f83d5b6ec276
Merge "Check for IPv6 file before reading"