Bug 1136751 - drift detection interval for individual drift definition is *always* ignored and value from agent config is used

Fix issues with previous commit.

Prevent the DriftDetector from running in infinite loop:
* when the queue is full of disabled schedules
* when a previous snapshot file exists

(cherry picked from commit 487cc7c9b3640f4fca960fd4665763052fd5303e)
Signed-off-by: Thomas Segismont <tsegismo@redhat.com>