adaptve status polling

In this blueprint, we propose nova should try to predicate a expected
time to finish an operation, with taking input load into account, adapt
 polling interval based on expected completion time and min/max_timeout.
which will:
* ensure system functional as designed in most of the time
* reduce the RESTful communication in between sub-systems
* ensures consistent end user experience
* overcome the shortcomings in RESTful system, while still keep RESTful architecture
  style intact