Being more defensive around the use_ipv6 config option

fixes bug 1175725

Will allow for the flag to be set to filter out ipv6 if
configured but will not add ipv6 to template if ipv6 is not
configured in quantum. This will allow spwan to continue
working in the case where ipv6 is not configured but flag
use_ipv6 is set in nova.

Change-Id: I26ebdff81efcdeff827521cec247d2c443a5e0ad
Merge "Being more defensive around the use_ipv6 config option"