This a follow up for change Ic8783053778cf4614742186e94059d5675121db1
and corrects the 'image_property set --property' arg format in the
hw_machine_type doc. Newline formats in the nova-manage CLI doc is
cleaned up to be consistent throughout and unnecessary () is removed
from the ImagePropertyCommands class.
Related to blueprint libvirt-device-bus-model-update
Change-Id: I5b67e9ae5125f6dad68cae7ac0601ac5b02e74b3