Error code for creating secgroup default rule

Trying to create secgroup group default rules
that already exists gives HttpBadRequest error,
but it should raise HttpConflict.

So updated the code to HttpConflict error.
Closes-Bug: #1374320

Change-Id: I93c6eb7fb96e7109404206367ed81495a4724296
Merge "Error code for creating secgroup default rule"