allow to dynamically set the "scaling_adjustment" for OS::Heat::ScalingPolicy per workload.

Issue in current OS::Heat::ScalingPolicy:
In OS::Heat::ScalingPolicy, there is a property named "scaling_adjustment", which is pre-set in hot template. There is no chance for
a system to dynamically set this property according dynamic workload. That is to say, only "scaling_adjustment" number of servers
will be scaled for current ceilometer alarm. If another server is needed, then it should wait another ceilometer alarm after
cooldown time. However, during the cooldown time, the scaled system still cannot handle the over load.

Suggestions:
In order to meet this circumstance, it might be reasonable to allow a dynamic "scaling_adjustment", which is dynamically
set per the incomming workload. For example, if incomming workload needs only one extra server, then "scaling_adjustment"
is set to 1, if incomming workload needs three extra servers , then "scaling_adjustment" is set to 3.

More think:
If dynamic way to set "scaling_adjustment" is allowed, it will also be useful when auto scaling worked with AI or ML.
And it can improve the ability to response changes in system.