constants: confd query types

Initially confd will support only two queries:

CONFD_REQ_NODE_ROLE_BYNAME
Given a node name, return its role.

CONFD_REQ_NODE_PIP_BY_INSTANCE_IP
Given an instance ip, return its node primary ip.
This rather weird query is the basis for ganeti nbma lookup.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>