* Change dns backends and pools.yaml to use
ipv6 addresses not wrapped in brackets.
* Changed MySQL backends to use MYSQL_HOST.
* Fixed bug in pdns4 driver not allowing
masters to use an ipv6 address.
Depends-On: https://review.opendev.org/#/c/678324/
Change-Id: I8f469e732618732f5417c1b5e6c2049eaa83ea78