Make HTTP keep alive configurable in relayd
To allow avoiding situations where the client keep alive is longer than the server, and some requests fail because the server closes the connection during the client request, which fails with:
oct. 12 21:31:14 relay rudder-relayd: ERROR relayd::processing::inventory: output error: HTTP error: https://server/inventory-updates/1634074274.912600401.sign: connection closed before message completed
We can also configure a lower default. 2s is below previous (15s) and current (5s) default value in apache httpd.