engine: Set MoM policy when host comes online

This patch adds the necessary backend infrastructure for enabling
and disabling ballooning feature on the hosts.

Change-Id: If5c73939b666228822e01fe51be3736d4d18ece5
https://bugzilla.redhat.com/show_bug.cgi?id=967572
Signed-off-by: Martin Sivak <msivak@redhat.com>