engine_page: Remove Retrieve Certificate button

ovirt-node-plugin-vdsm should popup to users the certificate
when they register via https to accept it or not.
This patch will add such schema and will remove the unneeded
Retrieve Certificate button.

Change-Id: I9f0e47242380c511e8ed672eeb0f541616696f8e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1081042
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>