Add required TargetGroupName to DBProxyTargetGroup
Add required TargetGroupName to DBProxyTargetGroup

Cloudformation seems to throw an error when a DBProxyTargetGroup lacks
the TargetGroupName property.  The Cloudformation documentation for the
DBProxyTargetGroup resource has no reference to this property, but the
examples on the page for DBProxy shows the TargetGroupName being set to
"default" in the DBProxyTargetGroup resource.

cloudtools/troposphere,cloudtools/troposphere