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
(cherry picked from commit 0c963f6140237d8d3f361c6aaa7d2a6b656504db)
Merge "Check for IPv6 file before reading" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack/quantum into stable/icehouse