Example Implementation for Nova Scheduler Periodic Security Checks

Provide common interface that must be implemented by a check to be considered pluggable according to the blueprint specification at https://blueprints.launchpad.net/nova/+spec/periodic-security-checks.
This example interface implementation might be re-used by the developer to implement his own checks. This example interface will use OpenAttestation service to perform static file integrity checks periodically.

Note: This is an example implementation for the feature specified at: https://blueprints.launchpad.net/nova/+spec/periodic-security-checks