Virt driver change to use API retry config value

A hardcoded value of 10 seconds was being used for the sleep time
between looping ironic API calls. A TODO was left to use a configurable
value. This value exists in CONF.ironic.api_retry_interval so we
should use that.

Change-Id: Ieca4c987b765e226f3921efc24f0ec051f0b1094
Closes-Bug: #1314638
Merge "Virt driver change to use API retry config value"