Change NodeLocked status code to a client-side error

Currently, the NodeLocked exception's HTTP status code indicates a
server-side failure, resulting in WSME logging it as such, and this
is leading to random failures in our CI tests. While NodeLocked is
a valid issue which should (currently) be exposed to the client,
it is not a server-side error and does not warrant a 5xx status code.

This patch changes the HTTP status code for this exception to
409 CONFLICT, causing WSME to log it as a warning. This is not ideal,
but it resolves the immediate issue (random CI test failures).

A longer term solution is being drafted at:
https://review.openstack.org/94923

Change-Id: Ied9dcf9bbcb505177adc57d7d8fe06e4096ff488
Closes-Bug: #1321494
Merge "Change NodeLocked status code to a client-side error"