Update devstack default to only use IPv4
Update devstack default to only use IPv4

Devstack was recently changed to use IPv4 and IPv6 (dual stacked)
as default.

Change-Id: I17221d2199f29af2bb5c762df79a68af3ca9ca34
Fixes: MNP-118

yamt/networking-midonet,yamt/networking-midonet