Rework policy check logic for built-in policy types

We classify all Senlin policies into two categories:

1. Built-in policy: for this type of policy, currently including scaling policy, lb policy, deletion policy, placement policy, their check timing are fixed:
e.g for cluster_scale_out action, built-in policies should be checked in following sequence:
scaling policy --> placement policy --> action execution --> lb policy

for cluster_scale_in action, built-in policies should be checked in following sequence:
scaling policy --> placement policy --> deletion policy --> lb policy --> action execution

So we decide to fix the priority of these built-in policies with the following value:
---------------------------
scaling policy: 100
region placement policy: 200
zone placement policy: 300
deletion policy: 400
lb policy: 500

2. Customized policy: for this type of policies, developer is allowed to customize their priority attribute to decide their check timing;