Fixesdocker/cli#4648
Make it easier to specify IPv6 addresses in the '--add-host' option by
permitting 'host=ip' in addition to 'host:ip', and allowing square
brackets around the address.
For example:
--add-host=hostname:127.0.0.1
--add-host=hostname:::1
--add-host=hostname=::1
--add-host=hostname=[::1]
Signed-off-by: Rob Murray <rob.murray@docker.com>