Make Confd client usable for testing

Allow the Confd client to be able to connect to an arbitrary server
instead of just the real one running on a cluster.

This is meant to be used for testing.

If a data collector using the Confd client is tested with shelltest, it will
need to receive through the CLI the address and port of the server, and also the
FQDN for which information is being requested. In normal usage it will likely be
the name of the node the data collector is running on, but for testing we need a
way to explicitly specify it so that it corresponds to the one used inside the
configuration files that the test confd server is reading.

So, optConfdAddr and optConfdPort and optNode are provided by this patch, even
if not used, because they are an important part of the modifications for making
the Confd client testable.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>