Refactors code in OpenStack plugin

Continues refactor process on OpenStack metrics plugin.
Delegates exception handling to the main metric handler,
removing exception management from specific function calls.
Extracts response generation code into single method.
Adds 'status' param in message responses where it was missing.
Modifies tests accordingly.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: I6ad27734f37708f5ccbef13e322723225545db4e