* guix/ssh.scm (remote-authorize-signing-key): New variable.
* gnu/machine/ssh.scm (deploy-managed-host): Authorize coordinator's
signing key before any invocations of 'remote-eval'.
(deploy-managed-host): Display an error if a signing key does not exist.
* doc/guix.texi (Invoking guix deploy): Remove section describing manual
signing key authorization.
(Invoking guix deploy): Add section describing the 'authorize?' field.