RPC: Add a new client type for DNS only

This patch moves the “call_version” to a new RPC client definition and
then adds a new runner using the DNS resolver for getting the host
address.

The standard “BootstrapRunner”, where the call was before, tries to
resolve node names using ssconf first, which doesn't work properly when
re-adding a node with a new primary IP address.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>