Consider number of available instances in deas before starting app.

Even when dea allows users to set maximum limit of app intances, CC still doesn't consider the number of available instance in deas when launching an app.  This change modifies the CC's behavior so that it selects only those deas that have the available instances for the app.
remove instances paramter from has_sufficient_instances? method
removed only_with_available_instances method from dea_pool class