Added backup notifications

Reasons:
 - Trove should emit notifications when resources are being used.

Changes:
 - added backup usage notifications:
  - trove.backup.create.start|end|error;
  - trove.backup.delete.start|end|error.
 - unit/int tests proposed.

Partially Implements: blueprint ceilometer-integration

Change-Id: I8e7dbdb82e4a200f338c4273eeacbfc7d87c603f