Gracefully handle duplicate rule creation

Gracefully handle duplicate rule creation

Previously, creating a second bandwidth limit rule for a policy raised
an uncaught exception, which eventually caused 'ServerFault' on the
client side. This patch replaces this exception with a NeutronException
which leads to a more correct 'Conflict' error instead.

Note that the code is implemented in the base object class. This means
that future versioned objects will also feature this restriction if
their database implies that no duplicate entries can be created.

Change-Id: I882d60843e1e651f3f9754746ac670f499431466
Partially-Implements: quantum-qos-api