Add support for non-TXT DNS lookup

[cdprete]

Hello.

I've an Eureka server deployed in OpenShift and I was trying to use the DNS settings which are provided to register dynamically the instances of the server by querying an headless Service.
After some debugging, I found that the projects queries specifically only TXT records (the txt. prefix is hardcoded) and that it necessary requires the <region>.<datacenter> prefix.

I know it's not ideal but, would it be possible to allow to define a default service that queries the specified DNS name without all these restrictions?
The type of the datacenter in the configuration (MyOwn vs others) may be used as a discriminator to decide how to perform such a query (i.e.: if it's not MyOwn, fallback to the TXT lookup as it is today).

Kind regards and thanks.