agent: enable global maintenance mode

Setting the maintenance flag ("maintenance=1") in the cluster-wide
shared metadata block will now cause all aware HA agents to ignore the
HA VM state, allowing the admin to perform arbitrary operations on/with
the VM.

To facilitate setting/unsetting maintenance mode, the HAClient class has
a new method:

  HAClient.set_global_md_flag('maintenance', <true|false>)

The boolean argument can be anything typically parsed as a boolean,
such as True, 'true', 'yes', 1, False, 'false', 'no', 0.

Change-Id: I796d768e197c5fe73646e67297714392bc6e8201
Bug-Url: https://bugzilla.redhat.com/1015724
Signed-off-by: Greg Padgett <gpadgett@redhat.com>