provide mechanism for disabling certain default health checks

[miabbott]

The opinion around #117 is that removal of the health checks is too coarse and severely limits the usefulness of greenboot as a whole.

Instead, let's investigate a means of enabling/disabling the default health checks.

[7flying]

• we need to give the user the ability to choose whether they want the default health checks enabled or not (all of them, some of them etc), given that they may have the default-health-checks RPM installed