de-hardcode adjustment type strings in autoscaling
de-hardcode adjustment type strings in autoscaling

The scaling policy adjustment types were hardcoded throughout
autoscaling.py: 'ExactCapacity', 'ChangeInCapacity', and
'PercentChangeInCapacity'. These are now constants at the module
level.

Change-Id: I0bd610e25bdba8ab13daf94b508934dca7e30c00

redhat-openstack/heat,cryptickp/heat,rdo-management/heat,openstack/heat,noironetworks/heat,maestro-hybrid-cloud/heat,pshchelo/heat,pratikmallya/heat,takeshineshiro/heat,cryptickp/heat,steveb/heat,pratikmallya/heat,rdo-management/heat,dims/heat,maestro-hybrid-cloud/heat,dragorosson/heat,dims/heat,pshchelo/heat,srznew/heat,srznew/heat,rh-s/heat,miguelgrinberg/heat,redhat-openstack/heat,steveb/heat,gonzolino/heat,gonzolino/heat,cwolferh/heat-scratch,takeshineshiro/heat,openstack/heat,jasondunsmore/heat,cwolferh/heat-scratch,dragorosson/heat,NeCTAR-RC/heat,rh-s/heat,jasondunsmore/heat,NeCTAR-RC/heat,miguelgrinberg/heat,noironetworks/heat