The current implementation of the `nomad tls ca create` command
ovierrides the value of the `-domain` flag with `"nomad"` if no
additional customization is provided.
This results in a certificate for the wrong domain or an error if the
`-name-constraint` flag is also used.
THe logic for `IsCustom()` also seemed reversed. If all custom fields
are empty then the certificate is _not_ customized, so `IsCustom()`
should return false.
The `nomad tls cert` command did not create certificates with the correct SANs for
them to work with non default domain and region names. This changset updates the
code to support non default domains and regions in the certificates.
