Expose the Openstack CPI wait_resource poll interval as a config item

Default 1 second sleep time on resource creation in the Openstack CPI could cause load/performance issues with some OpenStack installations. Making this configurable seems like a better option that enforcing a new default.

Change will keep the existing default if option is not set.

https://groups.google.com/a/cloudfoundry.org/forum/#!topic/bosh-dev/Q8_oZd9shJ8