Locking of conductor service process during DNS name resolving retries

For pure Python drivers (like iLO) vendor's library should resolve host names of management controllers (if they are used instead of IP's). Conductor service process can do this resolving frequently if local cache is not used. We set EVENTLET_NO_GREENDNS env variable for eventlet, and locking on one greenthread is possible on retries if DNS server temporary is not accessible. Typical retries time is 10 second (see man resolv.conf for detail). Eventlet resolver for older version does not support IPV6 at all, for current requirements a bug with IPV6 documented. We should switch ironic to "green" resolving in the future (versions in the requirements should not contain documented DNS resolving bugs).