auth/cephx: make KeyServer::build_session_auth_info() less confusing

auth/cephx: make KeyServer::build_session_auth_info() less confusing

The second KeyServer::build_session_auth_info() overload is used only
by the monitor, for mon <-> mon authentication.  The monitor passes in
service_secret (mon secret) and secret_id (-1).  The TTL is irrelevant
because there is no rotation.

However the signature doesn't make it obvious.  Clarify that
service_secret and secret_id are input parameters and info is the only
output parameter.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>