Fix keyword Halt Server to use --auto-approve flag

This change fixes the keyword Halt Server, used in tests heal02 and
heal04, to set VMs in error state before healing them. The issue
appeared with the new version of python-openstackclient, introduced when
the pip dependencies were upgraded.

Change-Id: Iabb76ac89c65bc1738dcbe9f0a01c765776e69be
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>