This commit makes sure that the plugin exposes right status in a
response body, and does not overwrite ERROR status until another
operation to the backend has succeeded.
This commit also changes NEC Plguin to use neutron constants instead of
OperationalStatus defined in this plugin.
Fixes: bug #1211319
Change-Id: Ic61b8e1b9d3f6c2be9567dd5a4606aa6d439c564