Rework policy check logic for built-in policies

Before action execution, whether an attached policy will be checked depends on its "TARGET" definition, including both action type and check timing. Developer is allowed to customized the TARGET attribute when developing their own policy type. However, for some built-in policy types, e.g. senlin.policy.loadbalance and senlin.policy.deletion, their target action and check timing are invariable, e.g. lb policy should always takes effect after new node(s) being added to cluster and before existing node(s) being removed from cluster. Therefore, for these policy types, we should embed their checking logic into corresponding action routine rather than check them in policy_check routine together with other customized policies.