Send HTTP exceptions in the format expected by neutronclient

Neutron client for the v2 API expects exceptions to have 'type',
'message' and 'detail' attributes. That is why they need to be
included in the body of HTTP Exceptions.

Change-Id: I70bd47977e42ad7bac760600329e9440452b74bc
Closes-Bug: 1355902
Remove invalid or useless initialization in test_type_vxlan

In test_type_vxlan, vni_ranges option is overriden with an invalid and unused
value, vxlan_group is overrriden but never used.

Change-Id: Ie1ce1d7a8ef54692d122276e371ec5efc2e68d9c
Merge "Remove invalid or useless initialization in test_type_vxlan"
Merge "Send HTTP exceptions in the format expected by neutronclient"