API: setHaMaintenanceMode command

New API for setting hosted engine maintenance mode.  This allows the
engine to send commands to the hosted engine agent in order to ease any
necessary maintenance tasks.  The following modes are supported:

local - allow maintenance of the host by effectively stopping the ha
  agent from performing any vm-related activity.  The engine vm is
  moved to another host in the ha cluster, and the host score is set
  to 0.  It is meant to correspond to vds maintenance operations
  performed by the engine.

global - allow maintenance of the ha system or engine vm.  All hosted
  engine state changes are halted for all nodes in the ha cluster,
  so that user-initiated changes to the vm or hosts are ignored.

This API improves usability for the hosted engine subsystem by enabling
changes in the engine ui that would otherwise require the command-line.

For more information:
http://www.ovirt.org/Features/Self_Hosted_Engine_Maintenance_Flows

Change-Id: Ic08c5edb0e9b8cc11eb70ef6a66301335c42aad3
Bug-Url: https://bugzilla.redhat.com/1053040
Signed-off-by: Greg Padgett <gpadgett@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23264
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>