core: Delay fencing until host storage lease renewal interval passed

Delays fencing execution until host storage lease renewal interval
passed in the case that fencing policy contains option to skip fencing
when host has live lease to storage.

Change-Id: I45da28ae8a3007ed124d145a303f6201474a0f1c
Bug-Url: https://bugzilla.redhat.com/1090799
Signed-off-by: Martin Perina <mperina@redhat.com>