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
Merge "de-hardcode adjustment type strings in autoscaling"