engine_page: use augeas instead of Management()

To manage values in /etc/default/ovirt we use Management()
which oVirt Node provides. However, for such task,
we should manage the values to be added into
/etc/default/ovirt directly in the Plugin VDSM to avoid
any possible issue involving other layer.

Change-Id: I063836cd373bcf81a4b3f07848d6e0240d4ef959
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1088875
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>