agent: enable local maintenance mode

The HA agent needs to be aware of local maintenance mode in order to not
run the engine VM on the local host.  This provides a method of setting
a local maintenance flag which is stored on the host and then read back
by the agent, which takes appropriate action to stop the VM.

A client method has been provided to set this mode:

  HAClient.set_maintenance_mode(mode, value)

Where mode is a constant in HAClient.MaintenanceMode, and value is
a value parsable as a boolean (True, 'true', 'yes', 1, False, etc.)

Change-Id: I60e62cd03daec812e16a3a5bbd2d3d896e41402d
Bug-Url: https://bugzilla.redhat.com/1015724
Signed-off-by: Greg Padgett <gpadgett@redhat.com>