Don't show "Service name will not be discoverable..." warning when DNS is disabled

In our Consul setup (v.1.11.3), we have a lot of services with names that include underscores (like `sth-with_underscore`). Also, we do not use DNS at all, which means we set DNS port to -1 via the `--dns-port -1` command line parameter.
However, a lot of DNS warnings are printed into Consul logs concerning the names that are not valid for the DNS discovery:

`[WARN] agent: Service name will not be discoverable via DNS due to invalid characters. Valid characters include all alpha-numerics and dashes.: service=sth-with_underscore`

#### Feature Description

I suggest not to show the invalid name warning if the DNS is disabled (the DNS port is set to -1).

#### Use Case(s)

The feature will be helpful for users who don't use DNS in their Consul setups.