engine_page: Add validation for cert remove

Before remove cert, we must verify if there is a cert.

Change-Id: If20ee6be62e8a4a39d5cb60580556c8fe29ce275
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1021157
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>