services: ntp: Ensure no double quotes are output to config file.

services: ntp: Ensure no double quotes are output to config file.

* gnu/services/networking.scm (ntp-server->string): Use the textual
representation of the values as printed by 'display' rather than 'write', to
avoid inserting double quotes in the generated config.
* tests/networking.scm (%ntp-server-sample): Add a comment and make one of the
options a string, to exercise the fix.
("ntp-server->string"): Move the expected value to the first argument.
("ntp configuration servers deprecated form"): Likewise.
("openntpd generated config string ends with a newline"): Likewise.