Change --disable-rollback option to --enable-rollback
Change --disable-rollback option to --enable-rollback

Due to bug 1131303, the engine default is changing to disable
rollback by default, so we instead need an option to allow
users to enable this functionality

fixes bug 1144630

Change-Id: I509e616e20a3e4750ffcefe6e72c52e5f60445bc

varunarya10/python-heatclient,larsks/python-heatclient,rdo-management/python-heatclient,citrix-openstack-build/python-heatclient,ecerulm/python-heatclient,tcpcloud/python-heatclient,varunarya10/python-heatclient,jasondunsmore/python-heatclient,ecerulm/python-heatclient,larsks/python-heatclient,citrix-openstack-build/python-heatclient,jasondunsmore/python-heatclient,rdo-management/python-heatclient,openstack/python-heatclient,metacloud/python-heatclient,tcpcloud/python-heatclient,metacloud/python-heatclient