This fix tries to fix the issue raised in 24391 about allowing
custom DNS settings to service definition.
This fix adds `DNSConfig` (`Nameservers`, `Options`, `Search`) to
service definition, as well as `--dns`, `--dns-opt`, and `dns-search`
to `service create`.
An integration test has been added to cover the changes in this fix.
This fix fixes 24391.
A PR in swarmkit will be created separately.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
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