Add VdsPowerDown command and use it in Power saving policy

This adds a new command that can shut a host down in a nice
way. It first tries to use SSH to execute /sbin/poweroff and
if that fails it falls back to power management and cuts the
power.

Change-Id: Iccb9064dcfc9c20901d32a550354d66bbb0cc863
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1035238
Signed-off-by: Martin Sivak <msivak@redhat.com>
restapi: Add the automatic_pm_enabled field to host.power_management

This field tells the engine to enable or disable automatic shutdown
and wakeup of a host.

http://www.ovirt.org/Features/HostPowerManagementPolicy

Change-Id: Ifbb099786fffbed918fe73a7256184f55ffef9f6
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1035238
Signed-off-by: Martin Sivak <msivak@redhat.com>